Just thought I would mention this, particularly for anyone who has non-school age children or for other reasons camp out of the main summer holiday months.
We went to a Yelloh site last year for a couple of weeks low season. When I got back I took out a loyalty card with them to which they added points for the stay we had just had. I have just booked to go away to another Yelloh site in late June/early July and the loyalty card gave me 4 free nights out of a 12 night stay. Bobby bargain.
Only problem is I think it only applies outside of July and August so is going to be less useful from next year when we are restricted to school holidays, although Easter camps or half term camps at Northern sites, might be an option, I guess.
Worth considering anyway if you go to the commercial sites - I believe Castels do a similar scheme, for example.