Duplay Inflatable Super 26ft Sports Arena £199.99 @ Fun4Kids
OK, so you're going to need a garden or communal area with 26ft plus of space (which rules out my 30ft garden really). Or have access to it - some parks allow temporary inflatables for private parties (makes sure you ask/get permission first). Or borrow some space! I have friends with a huge, enclosed grassed area communal to their flats that we've all used for largish-events, and a once a year summer picnic in the holidays to keep the kids amused. Maybe you're not lucky enough to live in a street with it's own square garden, but if you ask around you may find you know someone who does.
The sports arena provides an enclosed space for playing football or basketball. According to the manufacturer, the arena is made of "is made from durable PVC and double laminated nylon that is also double stitched." It stays continuously inflated with the electric air blower, included with the arena.
Thanks to Lottysdad08 at HUKD!