I've heard good things about these people (through www.londonwalks.com):

this is c+p'd from the site:

If you have only a few hours in London, a London Taxi Driver-Guide is perfect. Alan or Len or David collects you from your hotel in his black cab and gives you and your party a half day "whistle stop" tour of London's highlights. And for good measure, after the tour he'll take you to wherever you'd like to go in central London. Or how about this? If it's your last day in London, you could book Alan or Len or David for "a grand send off" - i.e., a Whistle Stop Tour and then afterwards they can take you and your luggage right to the airport! Can't be bad!! For further details, please contact Susan, Noel, Fiona or Mary on 020 - 7624 3978. Or email us

Maybe the reason you hear good things about these people from London Walks because they're relatives. Or scratching each others' backs. It's not unknown.

I'd rather have an opinion more certain to be free of bias.

I do NOT suggest this due to time constraints but since it was mentioned, the correct site for the original london walks (the one that gets good reviews here) is http://www.walks.co.uk/
the one referenced is one of the many rip off companies that siphon business off of the good rep of the real london walks company.

mk2 has a good point about the risk and the rushing around. personally, i would not do it but if you understand the risks and all that's your decision. i would not use the tube to get around as others have suggested...with such a very short time, why waste time underground? if you have one area to see, go to trafalgar sq, walk to the hungerford foot bridge (west side) which offers the best view of the london eye and westminster, etc. then walk around the west end, peek into chinatown, a little bit of soho. buy a book that details some london walks to plan your preferred route through the areas i describe. with so little time this will give you the best first view IMO.