Hi nwt,
Have you tried going to one of your local leisure centres?
We had a similar dilemma for my son's 4th birthday next month and Tooting Leisure Centre our hosting his party with a big bouncy castle, face painting and party games for 30 children totaling £180 for 2 hours. With that you will get 2 party helpers, so all you have to do is bring the food, cake and decorations.
You can also try your local library halls or the Kalos Centre on Endlesham Road.
Good Luck, and have a lovely party.
x