I had an email back from the Glastonbury office about whether I could take a car in with my van (you can't) and they confirmed that you can pitch a tent behind your van provided the two together don't take up more room than the pitch size. Can't remember how big that is but unless your van is massive, you'll be fine. And in practice, they don't even enforce the whole "behind rather than next to" rule in my experience. Just don't take the piss and you'll be fine. Welcome to the campervan posse!