THE OTHER night, under the Westway, a beggar asked me for money because he had HIV. It was the first time that has happened to me in London. We were in what is a gloomy place at the best of times: the eastern side of Ladbroke Grove, where older winos huddle at the foot of a wall covered in advertisements between the two bridges that support a motorway and a tube line. On the west side, under the bridge, the young crusties hang out round the public lavatory housed in a Portakabin.

Everywhere but under these bridges, Ladbroke Grove remains a place where people are solvent, not one where they sniff it. The large corner shop sells high-class snacks, such as cashew nuts and German beer. I had hardly any cash left from buying these when the beggar made eye contact just where it is least light under the bridge.

He seemed to be in his early twenties, with a squarish face neither handsome nor ugly, mouse-coloured hair cut short, and glassy blue eyes. It was obvious he wanted to talk as he moved towards me. I stopped. His breath smelt sweet and sour, like acetone.

'I've just been told - this afternoon - that I've got the HIV virus.' That was the first thing he said, and then he leant towards me. I wondered what I could say. I wondered if he would bite my shoulder. I didn't doubt his story for a moment: he was a man so obviously trying not to be frightened and, unlike most beggars, he was trying to communicate. He cared, I think, that I believed the story, although he told it disjointedly.

'All I want to do now is get drunk. The doctor told me this afternoon. Can you give me 20p for a can of beer?' His accent was junkie nondescript, a little hoarse.

I said I could give him 20p. I doubted that I had more cash on me, although I had pounds 1,000-worth of notebook computer slung over one shoulder. I put the carrier bag down and pulled out my wallet. While I fumbled to open it, wondering whether he would make a grab for it, he spoke to cover the embarrassing silence. 'I inject drugs, you see.'

There was a pause. Around the bend from the traffic lights is a hospice for Aids victims that Elizabeth Taylor had visited amid a scrum of photographers. It seemed very far away. 'And I mixed them . . . and . . . and - now I'm completely fucked.'

'Here's the 20p,' I said. There was something immensely reassuring about being able to give him exactly what he asked for. That may be why I didn't simply hand him the beer I had bought. The thought of doing so occurred to me only later.

He took the money without violence or greed, thanked me and headed towards the shops I had just left. I turned to watch him and he turned, too, to wave. 'I'll see you in hell,' he called, as if we were arranging a meeting in the Elgin down the road. 'I'm not going to heaven, you see. I'm going to hell.'

I waved back. I think I may have smiled, because 20p seemed such a very small sum to ask. No doubt he will ask for more as the story becomes something he can accustom himself to use. No doubt I'll find out when we next meet, wherever that may be.