We had similar issues with our previous van. In the end we purchased a piece of the thick memory type foam to fit exactly an top of the made up cushions, it worked a treat, although it was a pain to roll up and store.We got it through one of the sites advirtising on the net. if you decide to have a go at using the memory foam, shop around as the prices vary enormously.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.