Clive, I started to fill out the form then got stuck on an ambiguilty regarding support vehicle/campers which I've asked about by email, but had no response as yet, but it's early days.
When filling the form a floating total cost box follows one down the page, it starts at £25.00 if you click the toilet charge box the total goes to £35.00, and presumably totals up all your extra passes and such. So it's optional, I won't be ticking it as I have a camper.
Bernard