Sometimes it's the smallest things that catch you emotionally unawares.

There's a café at the end of my road that the past three decades have somehow passed by. It has one of those tills like an old-fashioned manual typewriter that makes a loud 'terr-ring' sound and throws open its cash drawer when you push down the keys. There's a timeworn Lyon's ice cream sign on the wall, a faded notice saying 'We accept Luncheon Vouchers', and a menu that includes rice pudding and custard for afters.

It's been run by the same couple for as long as I've been going there, which goes back to when Chris Smith and Margaret Hodge were councillors on the local housing committee. A squatter, homelessness campaigner and Labour activist in those days, I used to talk politics there with the regulars over sweet teas and bacon sarnies.

Both the proprietors and the regulars have grown old with the décor (although even 20-odd years ago the place seemed to have more than its share of OAPs discussing operations and whose funerals they'd been to recently). And I've gone there less and less, as the course of my life has taken me away from home more and more. In recent years I've hardly been there at all. An anonymous stranger, I'd imagined myself, sitting there unnoticed among the builders, the council workers from the office opposite and the pensioners fresh from their visit to the nearby Post Office.

I must have been in there just before leaving for Africa on my last trip overseas. And although I have no memory of it, I must have left behind a two-litre bottle of Diet Coke. (Yes, I know, but you can't expect me to boycott Kit Kat, Nescafe and Coke, can you?)

The reason I know this is that I called in there the other day with a serious starch and saturated fat craving to satiate. (With three slices, please. White bread, Mother's Pride, and cheap marge.)

I caught a glimpse of an elderly woman nudging her companion and nodding in my direction as I reminded myself what was on the menu (unchanged from when its 1970s-style red mock-leather holder was new). A few hushed words were exchanged with the woman proprietor, who disappeared into the back room before returning a few minutes later to present me with my bottle of Diet Coke.

It was more than six months since I'd left it there, tucked out of sight under the table where I'd been eating. One of the elderly regulars spotted that I'd forgotten to take it with me, but was no longer fleet enough of foot or loud enough of voice to alert me to the fact before I'd disappeared up the road. She'd given it to the café proprietor for safe keeping, and now, many months later, she recognised me on my return and reunited me with my bottle.

I don't know whether I was most touched by the fact that someone had remembered me amidst all the tumult and turnover of the ever-altering metropolis that is London; or that they had kept this bottle for so long in expectation of my eventual return. Either way, it made me feel a part of a community that I had only half recognised as existing at all.

These are the people for whom, every spring when I am in the country, I put out new pots and window boxes at the front of the flat. I do this even though, every spring almost without fail, eventually they get stolen, because I would rather share other people's pleasure in them for a short while than keep them to myself at the back of the flat, where they can't get stolen. Senseless and inconsequential though all this may seem, getting my bottle of Diet Coke back felt like a victory over everyone who would reduce human existence to a selfish scramble for survival.

Anyone who has spent any time in the Poor World will know that the poorest western visitor is wealthy beyond the wildest dreams of most of its inhabitants. It's one of the reasons why, whatever immigration controls we may decide to impose for pragmatic reasons, there is no conceivable moral case against 'economic migrants'.

Sometimes, though, even the most committed socialists can get frustrated when people seem to fail to do all they can to help themselves in their own countries.

I have been involved in trying to set up a website that would enable local people involved in sustainable tourism to publicise their own ventures without having to go through intermediaries, who then cream off most of the profits. Some of my contacts in Kenya were supposed to be getting back to me in January with details of the campsite and other initiatives they operate in the Maasai heartland on the Kenya-Tanzania border.

I had become increasingly frustrated as a result of hearing nothing from them, until I received an email this week. 'Sorry for the delay,' it said. 'We couldn't email you because the business has been terribly down. We didn't get even a single [visitor] since November and I haven't had the money to get into Nairobi for emails.'

Sometimes it's the smallest things that catch you emotionally unawares.