Seldom before had I seen Holmes so deeply wrapped in that impenetrable concentration which his close associates knew as 'sulking'. For two days he had sat wreathed in the smoke of his foul-smelling briar and staring, with room-to-let eyes, at that bit on the back of your hand where the hairs grow right up towards your little finger but for some bizarre reason don't touch the rest of your hand, although what it is about evolution, prehistoric man and the brain's survival instinct that makes it so vitally important that we keep our smallest pinkie warm when the rest of the digits are freezing in the coming ice age the Lord alone knows and you're not telling me it has anything to do with scaring off sabre-tooth tigers. But that's another story.

Only once had Holmes surfaced from his reverie. Keeping vigil over my companion, I had fallen into a deep slumber only to be awakened by an eerie squeaking and rustling, as if the sinews of a scrawny cat were being dragged across its feathery bones. I recognised the sound at the instant. "Holmes, must you always do your press-ups at this hour?"

His wan visage raised itself from the freshly Bex-Bisselled hearth rug. "Givus back me bloody violin then, ya old get," he snarled, then laughed the way men do when caught half naked on their elbows.

But now, as he reclined, eyes at half mast, in a pall of smoke thicker than the average Gary Numan concert, my mind replayed the eventful events that had prompted this cerebral freeze frame — the hurried visit from Inspector Bloom, the breakneck hansome cab ride to a deserted wharf, and the archway where a dying finger had scrawled in blood, the single word 'MIDI'.

In the puckering gleam of the inspector's bright red nose, Holmes had shivered. "Thank you, Watson, write that one in the book for bo-bos playtime. Now inspector, do you notice anything strange about the finger which left this warning?" The Inspector waved his large head from side to side, so clanking the big metal bell tied around his chin and alerting the Station Shepherd to his whereabouts.

"Why Inspector, do you not see that the finger which wrote this message has been made an example of in order to terrorise other men who might try to speak out. In accordance with certain strange eastern rights, this finger has had its entire body removed!!"

We saw it then, cursing ourselves for our slowness. At the bass of the archway lay a single index finger and no trace of a body. "Is this the work of Jack the Interface?" stuttered a Sergeant. "No, worthy constable," snapped Holmes, "you're thinking of the fiend who has been strangling prostitutes with family planning prescriptions — Jack the Rubber. I fear this is the work of a far greater evil. We must tread very carefully gentlemen, or the cleaners will give us that dirty look they reserve for people who have been working late in the office and who have to walk down the edge of the stairs so they don't stand on the bit she's mopped."

The Inspector and I exchanged knowing looks. Holmes was on to something.

The next few hours were filled with feverish activity. We visited a dozen musical instrument shops in the vicinity of the docks, always asking the same question. "Have you got my soddin' violin you scabby old bat?" But each time the shop owner would shake his head apologetically and Holmes would fist it for him. Yet slowly, a picture began to emerge of this shady underworld of music retailing into which we had stumbled. These were doughty tradesmen living under a veil of fear. And they all had glass jaws.

Finally Holmes tried a different tack. "Wait here Watson, I shall be gone but a few moments." I stood outside for 15 minutes, but when the sign still said 'engaged' I gave up and pressed on to our final call of the day. Approaching the doorway I was brusquely pushed aside by a strange figure wrapped in black. The man, for man I presumed it to be beneath the flowing shoulder length locks, more heavily greased than a Honda 250, had the wispy beginnings of a light yellow moustache, and matching growth on the underside of his chin only. I could have pinpointed his last wash by checking through the weather records for recent floods, and as he shambled towards the counter, only the dull clink of Gary Moore badges on his lapels contested for the ear's attention with the rattle of his beads. It was a Belgian heavy metal fan.

The manifestation approached the shopkeeper. "Please, I am musician and keyboard player. Can tell me, what is a MIDI?"

I was beside myself with excitement. Where was Holmes? This could be the vital clue we had been looking for. In the tradesman's eyes there was the merest glimmer of suspicion and puzzlement before he straightened and returned his inquisitor's gaze. "It is," he spoke, measuring his words, "one of those dogs that has very short legs and a body like a barrel."

"No," cried the Flemian metallurgist, "that's Jack the Russell". And the creature sprang back, throwing cloak, badges and wig to the floor to reveal...

"Holmes it's you... another disguise!!!"

"Yes Watson, I had my suspicions but this was the only way I could prove the link between MIDI and musical instrument dealers. You see," he continued excitedly, pointing a quavering finger at the tradesman, "they don't know what it means either!!"

But that had been hours ago. For a full two days Holmes had slumped in his favourite armchair, that great mind turning over the facts he had gleaned from his investigations, moving, unerringly towards a solution. Outside, the penny dreadfuls had already taken up the call. "MIDI Madman in Mutilation Mystery", "Murder Most MIDI", "My 30 seconds of fright in the same shopping precinct as the MIDI murderer", "Farthing shaky after MIDI conundrum."

At the sound of a sudden crack, like the snapping of a wholemeal digestive, I spun from the window to find Holmes on his feet. "Watson," he quizzed, "are you any good with your service revolver?". "Sorry Holmes," I stammered, "but I can only hit 1 in 10 of the villains I shoot at." "Just as I feared," replied Holmes, "a 10 per cent service revolver, never mind, it will have to do. In exactly one hour you are to go to the wharf where we found the finger, and wait for me. Foul things are afoot, Watson, and I fear for our rent."

At the appointed hour, as the clock struck midnight, I crouched by the docks and behind me heard an urgent whisper. "Watson, it is I. If you value your life, keep your eyes trained on that archway and do not move a muscle."

"Holmes, have you reached a conclusion on this wicked affair?"

"Yes Watson, though I was damnably slow. At first it was the terror amongst the music shop owners that fooled me, linked to the elements of sacrifice contained in that bloodied message which I took to be an old Masonic ritual warning. The dislocated digit was meant as a sign from instrument manufacturers to retailers that their livelihood — their musical dexterity — would be cut off unless they agreed to the manufacturers' demands."

"And the word MIDI, Holmes?"

"No, not MIDI, Watson, but two initials M, I, and then the last word was never completed. If you insert the missing letters the message should read 'Masons Impale Discounters'." "Brilliant Holmes. Dastardly clever of the villains in question."

"Dastardly indeed, Watson, because that is just what the evil minds behind this plot wished me to believe. But one clue would not fit into place in that oh so neat theory. Look again at the message on the wall, Watson, the downstrokes committed in a poor wretch's dying agony. Those marks, Watson, are almost an inch wide. No normal human finger could have made them. Not unless that finger had for years been placed under such pressure that the very skin itself had spread outwards, splayed into a flat, spatula-like shape, had been transformed into a new, wider profile, to become the fingers of... a double bass player.

"Yes, Watson, those are the daubings of a classical cellist, in fact, the last marks left by one Reuben Romney, my own long-time violin teacher and personal friend. It was he who stumbled upon the real reason behind the sudden flood of cheap synthesisers from the far east and that at the heart of the scheme was my own nemesis and most deadly foe. That is why he scrawled those letters, so I, alone, could decipher them 'Moriarty Is Doing It'.

"It is Moriarty's intention to turn every musical instrument shop into a synthesiser store, so that, at last, when all have succumbed to the digital keyboard plague, there will be nowhere I can go to buy a new violin. Though I have vanquished Moriarty in his criminal exploits, it is his bitter revenge that the last form of enjoyment and relaxation left to me will be taken away. And for that he required an accomplice.

"You, Watson, my most trusted colleague. It was you Moriarty bribed to steal my favourite Stradivarius while I was sleeping. So it is you, Watson, who must die. With your service revolver in your hand, crouched in the shadows, as though you were laying in wait to kill me under the instructions of your deranged supremo. The Police will find plenty of bullets embedded in the masonry — proof of your self-avowed incompetence with a pistol."

"No, Holmes, you cannot seriously mean to murder me, you must be joking."

"Oh no, my good Watson, I am most deadly serious... you must be thinking of Jack the Quipper..."