I have a new recommendation for a cheap minicab service.
I used
www.swisscottagecars.com for my last pair of trips. I had a mailing through the post from them and to be honest I assumed they were the same folk as Airport Direct. But they're not - they're £2 cheaper! £28 central London to LHR, and £34 LHR to central London (including parking and 40mins waiting time from wheels down).
They were only 5mins late for a 6am pick up (which is not only with the bounds of acceptability, but also much better than Airports Direct managed when I last used them). Also, and what impressed me most, was that I emerged from customs at 9.30pm on a Sunday night, off a transatlantic flight that had been scheduled to land at 9.55pm. And when I walked through (only 15 minutes
at most after the plane landed) the guy was there waiting for me, holding his sign.
I'll be using them in the future. Cars new enough, drivers pleasant, all properly licensed.