I was just in Mayfair and the best rates at any Marriott were appx 300+ GBP. Mayfair is fairly expensive, I would suggest not only PriceLine (do your homework 1st @
www.betterbidding.com &
www.biddingfortravel.com ) and stay away from Mayfair and the City itself especially if weekdays are when you need to be there. Going out a bit and using public transport to go back & firh will save you alot of Money
The hotels in Mayfair and close by are more on the Luxury end and wont be cheap