We just used to use 2 single ones and zip them together so that one was on top and was underneath ,we found it fine ,didnt get tangled and we were greatful of the extra layer :)
I use two flannelette double sheets, stitched together along the bottom and halfway up each side. They make a brilliant liner and are very cosy. The children have single sheets folded in half and stitched, but they don't work as well and they do get tangled in them.
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.