Many thanks for this: I only found the time to do it this afternoon ! I did better than the front seats, with £1.41, some small plastic toys, more biscuit crumbs than in a McVitie factory (mostly Mini Cheddars - we like our children to eat healthily), and a strange round phial of something liquid in a little plastic container with a snap-off end.
It was quite a heave to get the front of the seats to pop out, although it would have been easier without a four-yr-old who kept trying to join me in the back, whilst asking "What are you doing?" several times over.