The event can still go ahead, with several stipulations. 1. No spectators 2. Only one person per car 3. Probably no facilities being available (toilets or food) 4. No camping the night before.
As far as I understand the cost is still the same for the hire, which means we would still need to have 20 people to attend at £50 to cover the cost of the hire. If only 15 come, we'd be making a loss which isn't good as it would have to come out of my pocket.