In the Chinese astrological cycling system, 2009 was the Year of Being Cut Up By Buses, and 2010 is the Year of Being Cut Up By Taxis.
(2008 was White Vans, 2011 will be Motorbike Couriers, and 2012 Deadline-Chasing Olympic Construction Vehicles.)
The email for bus complaints is CUSTOMERSERVICES.BUSES@tfl.gov.uk. After two or three weeks you get a mostly standard reply saying we take this very seriously, the driver has been interviewed dah di dah.
It's easy to mock, but it does work, because two days later when you get cut up in the same way by the same bus route at the same junction at Trafalgar Square at the same time of day, it's a different driver.
Of course, the vast majority of cabbies are hard working, drive sensibly and safely, and I have no problem with them. They deserve respect: it takes a great deal of skill to negotiate your way through London's labyrinthine roadmap while texting your bookie and telling your fare how Nigel Farage should be running the country. It's only the tiny minority that are abusive, swivel-eyed bullies bent on intimidation. Say only five per cent of the 21,000 licensed black-cab drivers in London, in other words only... oh, actually that's over a thousand.
Anyway, one email line of complaint for taxis is Taxi.Complaints@pco.org.uk. However, they're only really concerned with licensing ('The Public Carriage Office has no legislative power to investigate road traffic offences committed by cab drivers'). You get the idea they're more concerned if the licence plate is displayed in the wrong font than if the driver ran you off the road to impress the hen party in the back.
But I see from my colleague Danny at City Cyclists that he's had some success by reporting bad black-cab driving to our friends in the Met at https://secure.met.police.uk/roadsafelondon/. No, really - and this was a genuine being-cut-up road traffic incident, not some outright very-serious offence such as taking a photograph of a pothole. I'll try emailing it next time I have to dodge a missile-taxi sequestering my ASL on a red in Whitehall.