Thanks so much, chaps.
Robbo, that's a great offer. It's a Pride GoGo and takes a pair of batteries (I can't actually see the size of them - I bought the thing second hand and have no manual but I'll undo the thing later and get a bit more detail for you.) I just want to be able to rely on not running out of power between Friday evening and Sunday after the race.
We managed OK on VdeV 3-day weekend but went home in the evenings and didn't have to get back and forth to the campsite.