The branches are bare, the sky tonight a milky violet. It is not quiet here, but it is peaceful. The wind ruffles the black water towards me.

There is no one about. The birds are still. The traffic slashes through Hyde Park. It comes to my ears as white noise.

I test the bench but do not sit down. As yesterday, as the day before, I stand until I have lost my thoughts. I look at the water of the Serpentine.

Yesterday as I walked back across the park I paused at a fork in the footpath. I had the sense that someone had paused behind me. I walked on. The sound of footsteps followed along the gravel. They were unhurried; they appeared to keep pace with me. Then they suddenly made up their mind, speeded up, and overtook me. They belonged to a man in a thick black overcoat, quite tall - about my height - a young man from his gait and attitude, though I did not see his face. His sense of hurry was now evident. After a while, unwilling so soon to cross the blinding Bayswater Road, I paused again, this time by the bridle path.

Now I heard the faint sound of hooves. This time, however, they were not embodied. I looked to left, to right. There was nothing.

As I approach Archangel Court I am conscious of being watched. I enter the hallway. There are flowers here, a concoction of gerberas and general foliage. A camera surveys the hall. A watched building is a secure building, a secure building a happy one.

A few days ago I was told I was happy by the young woman behind the counter at Etienne's. I ordered seven croissants. As she gave me my change she said: You are a happy man.

I stared at her with such incredulity that she looked down.

You're always humming, she said in a much quieter voice, feeling perhaps that she had to explain.

It's my work, I said, ashamed of my bitterness. Another customer entered the shop, and I left.

As I put my week's croissants - all except one - in the freezer, I noticed I was humming the same half-tuneless tune of one of Schubert's last songs:

I see a man who stares upwards And wrings his hands from the force of his pain.
I shudder when I see his face.
The moon reveals myself to me.

I put the water on for coffee, and look out of the window. From the eighth floor I can see as far as St Paul's, Croydon, Highgate. I can look across the brown-branched park to spires and towers and chimneys beyond. London unsettles me - even from such a height there is no clear countryside to view.

But it is not Vienna. It is not Venice. It is not, for that matter, my hometown in the North, in clear reach of the moors.

It wasn't my work, though, that made me hum that song. I have not played Schubert for more than a month. My violin misses him more than I do. I tune it, and we enter my soundproof cell. No light, no sound comes in from the world. Electrons along copper, horsehair across acrylic create my impressions of sense.

I will play nothing of what we have played in our quartet, nothing that reminds me of my recent music-making with any human being. I will play his songs.

The Tononi seems to purr at the suggestion. Something happy, something happy, surely:

In a clear brook With joyful haste The whimsical trout Shot past me like an arrow.

I play the line of the song, I play the leaps and plunges of the right hand of the piano, I am the trout, the angler, the brook, the observer. I sing the words, bobbing my constricted chin. The Tononi does not object; it resounds. I play it in B, in A, in E flat. Schubert does not object. I am not transposing his string quartets.

Where a piano note is too low for the violin, it leaps into a higher octave. As it is, it is playing the songline an octave above its script. Now, if it were a viola . . . but it has been years since I played the viola.

The last time was when I was a student in Vienna ten years ago. I return there again and again and think: was I in error? Was I unseeing? Where was the balance of pain between the two of us? What I lost there I have never come near to retrieving.

What happened to me so many years ago? Love or no love, I could not continue in that city. I stumbled, my mind jammed, I felt the pressure of every breath. I told her I was going, and went. For two months I could do nothing, not even write to her. I came to London. The smog dispersed but too late. Where are you now, Julia, and am I not forgiven?