Made in Britain...

There are two moments in Patrick Keiller's London I'll always remember. One made me explode with laughter, the other with pain.

The first - at the Trooping of the Colour - was the sight of a sergeant major swivelling a giant pair of wooden dividers, meticulously measuring the spaces between the guardsmen. The obsessional madness of this moment might have been crafted by Lewis Carroll. The second - outside a polling station on the day of the general election - was the sight of a willowy blonde exiting with two male companions. They exude privilege and power, and seem to know already that a Tory landslide is in the bag. They could be outside their club in Pall Mall as they pause on the steps and laugh. That hurt.

I hasten to add that this film mosaic is 84 minutes of memorable moments. Why do I resist calling them scenes? Concorde rising like a heron over a row of miserable houses close to Heathrow; the big boots of Bomber Harris as his statue is unveiled; the row of TV news-gatherers outside the Commons at night interviewing a row of politicians - each in their own pool of artificial light. I'm told all these moments were filmed without sound; yet you can hear the sergeant barking into that ear under the bearskin, and the rhetoric being pumped into our homes via those cameras.

One function this film fulfils is to remind us of the numerous wounds inflicted on London. As it jolted my memory, it made me realise how our governors have learnt that waffle is a secret toxic agent. If pumped out in sufficient quantity, it will eventually clog the minds of the electorate. They've learnt that in the avalanche of news, their own senseless vindictiveness and destructiveness are soon forgotten. It's sad that we don't have the same built-in facility as a computer to "autosave" - that regular moment when the machine decides to stop and quietly digest what it has been fed.

Keiller's ability to be in the right place at the right moment is comparable to that of Cartier-Bresson. His eye is impeccable - and witty. A sign for the Magritte Exhibition pops up while we're contemplating the vast sums of our money spent tunnelling under the Thames to provide an umbilical cord between the MI5 building and that of MI6. Can anything be more surreal than that? Later, a decaying shop housing a spiritualist appears just as we're being told about Conan Doyle's use of Vauxhall in his fiction - a fact, but also a sly reference to his interest in spiritualism. While Keiller weaves his film with multiple strands from the past, he always brings it back to the present.

It's a work difficult to capture on paper, let alone do it justice. That's because it is a rare commodity, a film revelling in images and sounds, complex, intelligent, free of overpaid stars and a formulaic script. The film is full of ideas and yet hugely entertaining. Instead of names on the marquee, it has two great unseen characters that glue the pieces into a satisfying whole.

It is the creation of Robinson and the unnamed Narrator that makes Keiller's conceit work so brilliantly. While the Narrator, of course, enjoys a material presence - the voice of Paul Scofield - Robinson is conjured up from words and what he - they - we - are observing. But what manner of man must Robinson be to allow Keiller the canvas he needs? Here, we witness a spark of inspired inventiveness. He makes him - I'm sure Robinson would approve of the term - a queer. We're even told that he and the Narrator once had "an uneasy, bickering sexual relationship".

It's worth remembering that queers of Robinson's age know what it's like to be pariahs - outsiders - subversives. His perception of London as a city of secrecy - a commodity much loved by those politicians and financiers with their hands on its windpipe - proves to be more acute than most. But not without another coup by Keiller - the character and casting of the Narrator.

The Narrator is a ship's photographer. We see the luxury liner on which he has arrived being towed helplessly by a small feisty tug towards Tower Bridge. The liner is full of rich passengers he must have been observing while his old friend Robinson, a part-time teacher, has been exploring the four corners of London seeking to understand its "problem". The Narrator has been away - cruising - for seven years. His diagnosis of the decline may be even more acute. He, too, doesn't disappoint. After all he does have the voice of Scofield - himself something of a recluse who, now and then, has to come out into the spotlight. No wonder he sounds as though he understands and respects Robinson.

I often find myself - usually from the top of a No 23 bus - fantasising about the fate of those shoals of foreign visitors drifting aimlessly along Oxford Street, past the exchange bureaus, shoddy shops and fast food restaurants, like lost souls who can't quite believe they've ended up in hell.

Now I have a new dream: that some of them may see Keiller's London, thinking it another tourist attraction or a way of avoiding the horrors of our capital city's transport system. Luckily for them the film is bigger and more profound than its title implies. But that's what you'd expect from Robinson even if - unlike Edward G - you never clap eyes on him. He happens to be the best guide in town.

'London' and 'Robinson in Space' are released this week as a double DVD by bfi Video, priced £19.99. 'Independent' readers can buy it for £17.99 (inc p&p). Call 020-7957 4767