Right, I've just phoned them up and they're fully booked during the day, have been for ages apparently. The earliest day time barbecue we can have is Saturday 20th September!
However, we can book the evening barbecue for the 24th of August if we want. 6pm - 11pm on a Sunday is going to be a bit late for anyone with work on the Monday, especially those travelling. So what shall we do?
1) Forget the barbecue, we can have a picnic or raid a burger van instead. (There is a cafe on site)
2) Book the evening barbecue, still have everyone turn up during the day but wait until the eveing before having anything to eat.
3) Find out the name and address of whoever has already book the barbecue and send the 'boys' round to convince them to cancel!
4) Hijack a Nova on the way and use it's boot as a barbecue.
Despite 3 and 4 being very tempting, I vote for number 1. I expect people could even bring those 1-use barbecue's as long as they also bring a brick to put it on so they don't burn the grass.