So you want to be on the radio? Then don't touch that dial because Jon Lewin is about to explore the world of the live session.

They're all at it: Peel does it twice nightly; his Rhythm Pal only does it once, even if he does go on longer; Peter Powell does it four times a week, though he repeats himself; Janice Long has two over the weekend, and even Richard Skinner gets in on the act during Saturday Live — he even does it in the studio.

Radio One is swarming with "live" sessions, almost 20 each week at the last count. And yet nobody seems to know how they happen, or what processes the groups are put through in the depths of the BBC before the tapes are handed to the DJs. Do they still use steam-driven microphones and hand-cranked tape recorders, or are their store-cupboards packed to overflowing with Emulators and Fairlights?

With these thoughts in mind, I dedicated a weekend to Studio 5, located deep within the labyrinthine interior of the BBC Maida Vale Studios. Saturday saw Ex Post Facto from Liverpool recording their third session, this time for a Peel prog. Their experience and ability meant another day for me in the studio, as I felt I needed some problems to give an insight into the workings of the BBC mentality. The following day's guests were Sunset Gun from Glasgow. I found watching these radio virgins far more instructive, as the mistakes they made were, according to producer Dale Griffin, fairly typical of inexperienced bands.

But how do you get there in the first place? Apart from turning right outside Warwick Avenue tube, is it a matter of demo tapes? I spoke to Mike Hawkes who, as Kid Jensen's producer, is mainly responsible for selecting groups for the programme.

"I hardly ever pick bands from unsolicited tapes — I just don't get the time to listen to them. More often, I see somebody gigging, hear a single, or read something in the press which will lead me to inviting the band in. Occasionally companies bring in tapes of new signings, which will get listened to... but it's always me who approaches the bands. Just recently we've had Jake Burns, Julian Cope, Carmel, and the Cure in; Madness have promised to do something. But I only ever book a band because I like the music."

Ex Post Facto keyboardist Frank told me that they were offered their first session by John Walters, following their single's exposure on John Peel's show. Sunset Gun are new signings to CBS, and secured their slot through the efforts of the company.

The major labels use radio work as a filter for new talent: the premise that "if a band gets on the radio, they must be all right" means less work for A&R people. Sessions also guarantee exposure both for a new signing like Sunset Gun, and for major artists like the Cure; Radio One gains the kudos of a Cure exclusive, and the label is assured airtime for its proteges. Ultimately the decision is, of course, the producer's, but it is difficult to imagine a group of the calibre of Madness being turned down on aesthetic grounds.

Different criteria are applied between shows, though. Sue Foster, secretary "Brian" of John Peel show fame, made the point that "bands tend to graduate up the evening, from John's show to Kid's and Peter's; which is what you would expect as they gain popularity from radio exposure".

Mr Peel himself is primarily responsible for his bands. How do you do it, John?

"It is, I suppose, appallingly random. Quite often it's just down to whether or not I like the people. Every band you book is representative of at least 20 or 30 others just as good, so it is arbitrary and unfair, but you just can't do it any other way... if you did choose bands on merit, you'd have hundreds on. Either that, or the same five over and over again."

I arrived on Sunday at 1.30 pm, about an hour after the group. In the studio itself, the bassist and guitarist were running through arrangements of the unrehearsed songs while writer and keyboard player Ross was placing disconsolately up and down with his hands in his pockets. It turned out that Sunset Gun had arrived with their newly borrowed TR909 drum machine with insufficient leads to connect it to the desk. Reasonably enough, they had assumed that the BBC would be capable of providing eight or nine jack-to-jacks, not realising that they only use balanced GPO leads, and carry no spares.

Since finding leads on a Sunday proved rather troublesome, we all settled down with styrofoam cups of the renownedly disgusting BBC tea to wait for what turned out to be an hour and a half.

Studio 5 is manned by three people for the purposes of these occasions. The producer is boss. Squeezing four songs (recorded and mixed) into 12 hours is his responsibility. It's up to him to nag the musicians into hurrying along.

And he must try and extract the best possible performance. Obviously diplomacy is a prime pre-requisite, but then so is the ability to bully sensitive (and insensitive) artists. Before starting his career freelancing for the BBC, Dale Griffin was better known as Buffin, drummer with Mott The Hoople.

The other personnel are the engineer, who is a BBC employee who knows and does everything, and his assistant, who does everything else.

By 3.15, the first drum tracks were being recorded. As the four songs went down onto tape, the first indications of a major problem showed themselves. Two of the numbers were left open ended, with almost six minutes of drums put onto tape. Dale stressed the point that time was already at a premium because of the late start, and having five minutes of song to work upon when the final mix would fade out at four minutes (maximum) was merely a further waste. It quickly became apparent that Sunset Gun were not as well prepared as they might have been.

Sunset Gun, who sound like a jazz-funk Prefab Sprout, have only three members — Ross, and the singing sisters Deirdre and Louise. Guitar and bass are provided by part-time members Jim and Simon; both are obviously excellent players (Simon also plays guitar with Incantation, and bass with the Ballet Rambert), but their unfamiliarity with some of the material proved something of a hindrance.

By 4.30, the last drum-track was finished. At the same time the day before, Ex Post Facto were being told that this was "very late" for a first run-through. However, their songs were much simpler to record, taking only two hours. Sunset Gun insisted on frequent dropping-in to patch up their backing tracks. Admittedly the end result was very smooth, but it was remarkably time-consuming. The rising air of tension in the studio was not helped by the absence of Deirdre and Louise (they eventually arrived at 5.45).

As the clock moved round towards six, the importance of the producer's role grew. It is up to him to balance the time available between backing and vocals, as too much concentration on instruments can pressure the singers into giving a poor performance later on, one which they will not have the time to rescue. Engineer Mike Engles: "Technical perfection isn't so important in radio work, as the tapes are only heard once or twice, not over and over like records."

6.05: with the drums, bass and guitar finished, the focus moved on to the piano, a Steinway grand that comes with the studio. Three mikes and a dab of AMS digital reverb give a beautiful warm sound that is matched by the quality of the playing. Ross is obviously excellent, and the control room relaxes at the knowledge that he won't take ages correcting his errors.

While this is going on, I paced around the studio poking and prodding the various pieces of equipment on display. The mixing desk is a 24-track in-line Solid State Logic, backed up by a REBIS outboard EQ, and a heady selection of other effects, including AMS reverb, an Eventide Harmonizer, a flanger, noise gates and all the other traditional studio hardware. Not satisfied with this, Mike Engles went out with a trolley to search the studios for more units to pillage. Even the old microphone-and-speaker-in-the-corridor for ambient reverb effect was employed.

Having an in-line mixer is a great advantage as it means that EQ and effects can be set up while recording is in progress, thus saving much time during the mixdown.

6.45: following the successful pianoing of all four songs, it was time for more revolting BBC tea while Ross fiddled around with his brand new DX7 looking for suitable noises. Both this and the TR909 were a product of CBS' advance, and Ross' unfamiliarity with the gear proved a definite nuisance later in the evening when overdubs were started.

As soon as Deirdre and Louise began warming up at 7.20, it was immediately obvious that the two were blessed with amazingly good voices. They harmonized, sang in unison, and counterpointed each other so perfectly that again the atmosphere changed. Dale's constant enquiry, "Are your headphone mixes OK?" was an indication of his concern for coaxing the best possible performance out of two already gifted vocalists.

At 8.30, the vocals for the first two numbers were down and double-tracked. By 9pm, the future single "Stay With Me" was finished. Egged on by Dale's soft-spoken diplomacy ("a bit more angst and you've got it!"), the girls sang almost perfectly. The next and last song and any repair work necessary to other tracks took a mere 40 minutes.

The SMPTE timing code on the desk is a great boon in locating drop-ins on the tape, a process that the computer control makes simplicity itself.

By 9.50, all the basics had been dealt with, leaving just the instrumental overdubs and the mixing. The script — a detailed breakdown of the songs into individual parts and arrangement — turned out to be rather less accurate than Dale expected, with the consequence that more playing had to be squeezed into even less time.

Ross headed back into the studio to find a suitable string sound on his DX7, asking through the talkback if Dale would tell him which was the silkiest setting. "The first" is the immediate reply before Ross even touches the keys.

10.10pm: more fiddling on the "stryngth" as it is described on the desk's VDU. An air of exasperation is building up in the control room as time sprints on, with both musicians and technicians aware of how little time will be left for the mixdown. A theory is advanced that this procrastination might be a by-product of Sunset Gun's recent sojourn in the studio at CBS's expense: it is difficult to readapt to working at speed after the relaxed atmosphere of free recording time.

"The basic spirit of these sessions is to play as live as possible," Dale told me. "Not only does it speed matters up, which is vital, but it also often gives a good feeling to the product. It gives our recordings an urgency that records sometime lack."

The constraints on the length of BBC recording sessions are mainly self-imposed. The studio is booked for a 12 hour period, though at weekends it is possible to over-run. Payment to the musicians is based on this (MU rates demand the groups are paid about £60 per head) as is payment to the producer — he doesn't get overtime, so it is certainly not in his interests to allow bands to exceed their 12 hours.

Obviously limits are flexible because the tapes have to be finished, but long stints in the studio tend to become counterproductive after a certain threshold is reached, and standards to decline.

Dale took great pains to point out that Sunset Gun's plethora of guitar and synthesiser overdubs ran contrary to the usual ethic of BBC work.

10.30pm: guitar dubs. Even if this method of working was unethical, the songs seemed to be shaping up well under the strain. The future single, "Stay With Me", received most attention, as it was plain that this was the arrangement that the group knew in most detail. I listened to a tape of the single version as scheduled for release, and in comparison with the BBC tape, it seemed a little bland. Despite the fatter drum sound and the presence of a horn section, it lacked the edge of Dale Griffin's production.

10.40pm: the last guitar part, a dead-ringer for an Ernie Isley solo, is laid down, and the band are banished from the control room as the desk is prepared for the final mixdown. Only leaving an hour for this is definitely not standard procedure, as 15 minutes per song mix hardly gives any leeway for altering effects settings between numbers, let alone experimenting with instrument levels. The work started around 11.15, and proved remarkably (and happily) simple. The advantages of in-line mixing desks made themselves felt, as did the Sunset Gun's single-mindedness — their insistence on multiple overdubs made the instrumental track lush and full, thus removing the need to worry about individual instruments' ascendancy and permitting a straightforward mix. The girls' superb voices carried the evening, giving the songs that hint of something extremely special.

Listening to the day's work (it was finished by 1am), the tension occasionally apparent in the studio had not obviously tainted the four songs that constituted the final result.

Should Sunset Gun eventually wish to release their BBC tapes on record, as others like Scritti Politti have in the past, it would be necessary to negotiate with BBC Enterprises for the sale of the tapes. As it is, the group will receive a repeat fee if the session is re-broadcast, and the Performing Right Society will collect their royalties as if for a record.

Four songs, recorded and mixed in less than nine hours, complete with multiple overdubs. I occasionally got the feeling that perhaps 24-track studios weren't quite intended to be used in this way, but given the end result, and the fact that the group are paid for the privilege, who would complain? (Except me, about missing my last bus home...)