Thanks Steven (phoebe) for your help
.I emailed NPA today and got this reply :
"You can park overnight in the campervan / caravan site.
This is an un-serviced site so I advise you bring your own water, etc. but there will be portaloo facilities.
If you wish to do so can you please complete the attached form and return so that I have record of your intention to stay there."
Also as you rightly said you must register your vehicle in advance and parking is free, generators are not allowed due to fire risk. So getting the wellies out for hopefully a decent couple of days.
Thanks again
.