We had a clown called Fizzie Lizzie five years ago for my dd's birthday party, and she was brilliant, and tough too, which is important if you're inviiting lots of five-yo boys. 020 7723 3877.
Otherwise try here for NW London ideas.
Since that first extravaganza I've gained confidence and tended towards DIY parties, which are actually fine as long as you know what to expect. We've hired the local 2o'clock club, which has a big garden, and you get all the little cars and bikes and stuff. We've started the children off with a craft activity (eg. making masks or party hats), followed by running around, followed by tea, followed by more running around, followed by party bags and home. Much much cheaper than the whole entertainer thing, and no more stressful, in the end.