Call the Keen Group (minicabs): 0208 875 4949They'll take your flight number and city arriving from, and call you as soon as you land, and pick you up from arrivals. Not the best of service (minicab after all), but relatively cheap, quicker than the Heathrow Express rip-off if you're going anywhere other than Paddington, and reliable (used them about 30 times, always turned up). All their cars are tracked on GPS so you can call up their call centre and they'll tell you where the driver is. Plus the far is transmitted to the driver's GPS from teh call centre so they can't rip you off.