I'm told by a non-caching acquaintance that Billing Aquadrome near Northampton is an ideal venue for this type of event. It is purpose made for large organised rallies and is used by many car and caravan clubs for there annual meets. No idea of costs or availability (often booked more than a year in advance I'm told) but may be worth a look. And before anyone suggests it, no I'm not volunteering as we are likely to be moving home around this time next year . I would certainly be more than willing to lend a hand though.