I have hired non-themed ones from jumpen around:
http://www.jumpenaround.com/They are great - no timed hire, you can have it all day, its all legit, no cash jobs, either direct debit, credit or debit card.
We have even had one left at our house all weekend as they had no other bookings on it and the kids has a ball (picked it up on the monday after a saturday booking as they were busy all weekend with the other castles but the one I picked was not otherwise booked!
I have used them now 5 times in all and they have been great.