I'm a cabbie and I live in Coventry, don't do posh ;D but a sheet, surely commonplace amongst campers?
Sleep-in-bags from decathlon are good. Don't know about self inflating mattresses though so can't compare.
Its easy to overinflate the mats and they are not as comfortable when you do that. If you are using one for the first time experiment with different pressures. I found that have them very soft was best.