When I returned home that evening I heard Sherlock practising his violin. Sometimes, as you might well know, he pretends to be terrible, scratching at the instrument, sawing at it as if trying to break the bow, not even noticing that he's making the most horrible sounds known to man. But this time he was actually playing it. I could see him even before I opened the door: he would be standing a little pompously by the window, sweeping the bow across the strings and producing magnificent music that nobody could possibly complain about.

However, when I pushed open the door I saw that he was not by the window. He was at his desk, scribbling away at what looked like manuscript paper with his violin across his lap.

Now Sherlock is not only an able and talented violinist but he is also a gifted composer, writing short works of the most incredible magnificence that are rarely played outside of 221B, which is a shame. This time he seemed to be embarking on something longer and bolder, for he had bought himself a book for orchestral scoring and was scribbling away on all the staves, writing for every instrument. He looked up as I entered; seeing that it was me, he went straight back to work.

'Tea, Sherlock?' I asked.

He gave a non-committal grunt that I understood to be a yes. Therefore I went to put the kettle on, and a minute later returned with two cups of tea, one of which I placed on the desk, glancing at the music.

'Violin Concerto No. 1?' I read with mild interest.

'Excellent reading skills,' muttered Sherlock, drawing a treble clef on the next line with a flourish and writing in notes so definitely that I did not doubt that the entire work was already in his head.

'I'd love to hear it when it's finished.'

He was so absorbed in his task that he didn't respond, but I knew that he had acknowledged the compliment in his own special Sherlock way – that is, he looked as if he had completely ignored it but the words would bubble up inside of him and please him more than he would let on. Because I had told the truth – I had a liking for Sherlock's music, and I knew that this work would surely be better than any that had come before.

I never heard any of the concerto before it was complete; evidently Sherlock practised it outside of my hearing, testing it out fully, making sure there were no mistakes or clashes or anything that might smear the sound of it. His music had to be perfect, I knew that much. He had thrown out entire pads of manuscript paper before now because he had written the wrong note or scored a piece in the wrong key.

But at last he was finished, and he told me so, which surprised me for he was not usually communicative about his projects. He sounded uncharacteristically pleased, and I agreed that very evening to listen to the whole work, which was scored for a full orchestra but of which I would hear but the violin part – the solo that rose above the rest of the music, taking all of the glory and the spotlight – in a word, Sherlock.

And he raised his violin to his shoulder and began to play.

Allegro vivace

The first movement was striking in its brightness and excitement, and was so upbeat that I could hardly imagine Sherlock penning it. It was filled with action and adventure, and I at once could place what it was supposed to describe: the flurry of a case, the intensity of gathering information and running around London. It was immediately metropolitan, conjuring up vivid images of our city, with its traffic and taxis, with its ginnels and back streets, reflecting the close relationship that Sherlock had with London, for he knew it better than anyone. I could see us dashing down a puddle-filled road, me a little behind him, having to dodge the greatcoat that flapped towards me and never knowing quite what was going on. And it became faster and more insistent, and I could tell that we were on to something, that Sherlock had caught hold of a trail and was following it so closely that it would become entwined with his very being; and then –

Adagio

And then, in stark contrast to the first movement, Sherlock's playing slowed right down, the last flurries from the mad dash still hanging in the air. The languid Adagio seemed to suggest Sherlock's other side, capturing the image of him meditating on the details of a case, leaning back in his chair with his eyes lightly closed, his lips pursed, his hands clasped – and woe betide anyone who disturbed his absolute peace. I could almost see Sherlock's thoughts, organised and methodical and yet so very brilliant, shining out from the piece as the case unfolded and became clear. His playing descended right to a ppp (or so the music dictated), as if to fade out, and then suddenly there was a spark, a flash of inspiration, an explosion of ideas, and I knew that the case was solved.

Allegretto appassionato

And he dived right into the unusual and rather spicy finale, which depicted the dénouement of a case and the confrontation with whoever it was behind it. The dark and terrible chords conjured up a dastardly and yet shadowy villain, one who eluded and evaded Sherlock for as long as possible before confronting him. There was tension, action, even a few gunshots – sharp notes played stringently and determinedly; and then it seemed that Sherlock had triumphed, for even in his playing he celebrated, smiling vaguely as he stabbed out the last few, bold notes, coming to a close on an overwhelming positive, suggesting the emotions that he never showed but must surely feel.

The last note lingered in the air for a long while, before dissipating and leaving the room in silence. Sherlock lowered his bow; I felt obliged to applaud, but he glared at me, so I didn't.

'That was really good,' I said honestly; I wasn't normally that into classical music, and I had never before then understood how anyone can sit through concerti that go on for half an hour or more, but I could genuinely say that this one had captivated me, leading me back through every case that Sherlock and I had solved together, taking me around London and back to Baker Street in a piece of music.

'It's supposed to be with a full orchestra,' Sherlock conceded, glancing towards the complete score that was still on his desk.

'Including taxi horns,' I commented, following his gaze. There beside the percussion stave was a note: Taxi horns – Gershwin?

'An American in Paris,'* Sherlock said, as if that was supposed to explain everything. 'That's the London part. It didn't sound right with just the violin. There should have been taxi horns. And there were supposed to be gunshots in the final movement.' He ran a piece of rosin down his bow with just a hint of a dejected air.

'But you did the gunshots with the violin, didn't you? The sharp notes – wasn't that what they were?'

'It needed gunshots,' said Sherlock somewhat sulkily.

I readied myself for a lurch towards the drawer where he kept his pistol, but thankfully it didn't come. Sherlock merely put his violin away and went back to the desk, scrutinising the piece once again, occasionally writing in new parts or scribbling out notes.

'It sounded brilliant. Really,' I said truthfully and emphatically.

A rare smile flickered on his lips for a split second. If I had blinked, I would have missed it.

'You should submit it, see if anyone will record it.' It was a casual statement but I truly wanted to see Sherlock gain some credit for something he had done, even if he didn't.

His lip curled, and he said nothing but I could tell he didn't like the idea. I couldn't quite work out why.

'Really, Sherlock, quite frankly you're always boasting,' I said. 'Don't go all modest. It doesn't suit you.'

He remained silent, and I decided that disturbing him when he was in a mood wouldn't be a good idea. I left him to put his violin-case in the corner of the room, fold up the music-stand and place it a little randomly on the coffee-table (I will never get used to Sherlock's idea of tidying up) and close the book with the music in it, stroking the cover a little. He was proud of his concerto. That I knew. But for some reason Hell would have to freeze over before –

'You don't want anyone else playing it, do you?'

He looked up. A glint in his eyes told me I had struck the right chord. 'What do you mean?'

'You're the only one who'll ever be allowed to play the solo, and if it's recorded or performed they'll choose someone else to play it, and you wouldn't like it. It's your work – the Sherlock Concerto. It's like it's part of you.'

I could tell that I had understood well his thoughts because he glared at me.

'I'd love to hear it as it was meant to be heard.'

A brief smile; his hand went back to the cover of the book. 'As would I, John...'

*An American in Paris by George Gershwin uses genuine taxi horns to conjure up the image of a busy city.