After a few more tries I managed to get the stuff to stay on the back window. It works, kinda anyway. I don't think I'm completely happy with it though and on our *adventure* this weekend gone Hubby says it's not the best. He says you can't see out of it good enough. And as you can still see inside the van from out the back I am rethinking the whole thing.
I did buy some other stuff to try but haven't put it on yet. It is a film of tint stuff, what I wanted to try in the first place but couldn't find an any of the shops I looked in.
I will report back when I do the changeover.