I recently hosted a 5th Birthday Party and used Dantastic Productions www.dantasticproductions.com to entertain the children, and want to recommend them. They kept 20+ kids' attention for the duration and were very friendly and reliable.
We had the party at Christ Church Hall, Christchurch Rd, SW19 which was an easy, basic, hall to deal with too if anyone is looking for venue ideas.
I've been to far too many kids parties in the last 12 months and Dantastic are without doubt the best. I haven't hired them myself, just seen them in action. However if you can, go for the actual Dan, not Matt. Matt is great, but Dan is awesome, the kids hang on his every word.