I'm not a big fan of the Heathrow Express and cab combo. I gave it another shot yesterday, as I was arriving in London midday. Even at that time, the taxi queue stretched a block. I was in line for a half hour. It was then a 12pd taxi ride to the Trafalgar.
Mini-cabs in London are much cheaper than the black cabs. I know some of the UK residents have posted reliable mini-cab services as has squeakr above.
[This message has been edited by SanDiego1K (edited 09-23-2003).]