moderate price - good location. Covent Gardens, soho?

What do you mean by 'moderate price'? And when are you planning to go? If it is during the summer and if you don't mind roughing it a bit, I'd recommend looking at Unite student housing, and the accommodations maintained by the London School of Economics - the locations are fantastic and it is very cheap, IMO. They are open to the public from early July to late September.

As above depends on when yo are going. I always use www.londontown.com. I like South Kensington for the transportatikon adn walkabilty of several places, also good services and lots of restaurants

We found the best deal by bidding on Priceline. We'll be spending 2 weeks in London in Sept. and got the Park Plaza Victoria London (bidding on the 4-star in the Westminster area) for $95USD a night plus tax. We used a coupon from the Entertainment Book that allowed us $10 a night off so the most common winning bid without the coupon is $105 a night. Go to www.biddingfortravel.com to see how to bid.

How many people? When? Budget? How long?

There - I can use as few words as the OP . . . .