The door is fixed!
We had bumped into this German couple at a few crags and then at a cafe where we got chatting. After a pizza and a few coffees we spoke about our rucksack bouldering mat and the fact that our back door was broken. Turns out Wolfgang is a bit of a mechanical whizz and after about half an hour he had the door open, as well as showing us what to do if it happens again. Huzzah!
So basically prepare for lots of smug van photos with both back doors open.