Put simply, recovery of overcharges can only be by way of either a Small Claim to the local County Court or set-off. Note that pitch fees are a private matter between individual occupiers and the site owner. The residents’ association plays no part in pitch fee reviews.
I would hope the Small Claim spoke for itself. However, if there are a number of occupiers and the same sums of money are sought by each occupier, I would speak to the local County Court staff and ask if in the circumstances, a ‘class action’ was possible.
As regards each occupier seeking to recover the same sum, write to the park owner spelling out clearly his wrong, giving him the opportunity to reimbuse the sum sought within a stated period of time, say seven days.
This presupposes there is no clause in the Express Terms of the agreement denying the application set-off. Advise the owner that if the sum is not reimbursed within that period, the next (and/or future) pitch fees will be reduced accordingly to recover overpayments. Set-off has its attractions as no cost is involved but the wording of notice to the owner is very important.