TWO FEET behind my left shoulder and a little above it, something fascinating must be happening. Strangely, I can't quite catch it, because whenever I turn round it stops and disappears. Yet the eyes of almost all the several hundred people who pass me every minute are firmly fixed on that spot, so something must be happening there. Indeed, it's so interesting that most of them seem not to notice my collecting tin.

It is a cold, sporadically damp day, and Crisis, the London-based homelessness charity, is collecting on the streets. It has always been hard to count accurately the homeless in any large city, but estimates suggest that more than 4,000 people in the Greater London area will sleep rough tonight - under cardboard boxes and newspapers; in derelict buildings, bus shelters and shop doorways.

That is only a small part of the story, a small part of the crisis. Many more will be, in the absurd terms that make up the language of poverty, fairly lucky: in hostels, night shelters and private 'hotels' that are no better than battery farms for people. They may have a roof for the night, but little or nothing else.

In fairness, not all those rushing past pretend to be looking at something just behind me. Some suddenly discover stains on their ties or develop renewed interest in conversations with fellow travellers. I can't help thinking that there is a promising career for me as a bank robber. All I'd have to do would be to carry a charity collecting tin with me, and passers-by would swear they were looking the other way at the time; the police would finally be forced to issue a photofit of the building next door.

I am standing at the end of London Bridge Walk, where the walkway from the main-line station meets the bridge itself: one of the city's busiest thoroughfares. A passing policeman, turning a blind eye to the fact that I'm 20 yards inside the boundary to the square mile of the City (which the collection permit does not cover), tells me that the daily human traffic across the bridge has been recorded at almost 100,000. I find this so hard to believe that I try a 'flow-count' for myself.

I give up at about 150, well before the first minute is up. This suggests around 10,000 pedestrians an hour, just from the station; and even if this intensity is maintained for only a few hours at either end of the day, it's quite an opportunity.

They may not all be rich, but they can all afford a train fare, and it hardly bears thinking about what could be achieved if each of them donated just a few pence.

The trickle of coins is miserable. In most places people are embarrassed to give anything less than a 20p piece; the average is more than 50p and pound coins are common. But here in London - where, despite relative decline now that the recession has finally reached the capital, people are generally considerably wealthier than in most other places I've collected - the meanness is staggering. During the lull between the rush hours, one chap comes out of the office block on the walkway.

'And what are you collecting for?' he asks, peering myopically at the writing on the tin for several seconds. Then he says knowledgeably: 'Oh. Homeless people. Yes, there are far too many of those, aren't there?' He digs around painfully in his trouser pocket before putting 10p in the slot, beaming benevolently. It is as if he feels there is a sort of optimum level beyond which all those without homes should be culled.

Crisis used to be called Crisis At Christmas, and remains best known for its 'Open Christmas', which offers respite and turkey to rough sleepers for a week or two around the Christmas period. The name was changed for the obvious reason that it was misleading. Christmas ends, but the crisis continues. The guests are put back on the street until next year.

If you telephone the Crisis head office in Whitechapel Road out of office hours, a recorded message pointedly thanks you for calling 'Crisis, the charity working for the homeless all year round'.

One woman smiles sadly and tells me she's having a bit of a crisis herself this year. Her expensively coiffured head bobs off into the crowd, pleased with its own wit.

I develop a sort of slow-motion dance to rotate in a gentle sweep, so that I can try to catch the eyes of people coming from either direction. I am halted in mid-spin by a stooped man reaching from almost behind me to push something into the tin. It is a tightly folded pounds 10 note. He rushes away without looking at me, leaving me to wonder whether he's just embarrassed by his own generosity or whether he's a landlord with a guilty conscience.

A lost-looking man in a long raincoat (it's quite wet now and I've come out without mine) asks me how to get to Guy's hospital. I give him detailed directions while waving the tin prominently. I move it gently and seductively as I point and gesticulate with the other hand. He thanks me and walks away. I hope he gets there too late and dies.

It's time to give up collecting and become visible again. I know this because I have reached the stage of actually hating those who can't see me. That is dangerous, of course, because to deny people's right not to give to a cause - however urgent - is to begin a process of logic that in the end can only lead to a denial of my own right to extravagant priorities. And we all have those, so it's time to stop. It's time to go home. Because I can.