For those who don't know, the MMM magazine publishes a helpful list of motorhome-friendly car parks and overnight stops (most not free, however!)
You can download it as a pdf file HERE.

(Note - this link takes you direct to the pdf to view on screen, which has rather annoying annimated page turns etc and isn't quite a full page online even in full screen mode as it displays in book form. However if you save the file to your device you can then just open and view it full size without animation in the normal way.)