Having tried everything from Cillit Bang to Cellulose Thinners trying to remove the stains from Willum's seats my daughter Debbie suggested that I try these "Magic Erasers" that she uses for her trainers.
Guess what?........ THEY WORK!!!
Used with plain warm water this little sponge removed the grime with hardly any effort at all. The photo shows the back of the passenger seat with the bottom half cleaned. There are several brands but they are all more or less the same thing. I got mine from Tescos, four for £1.00. and have ordered 30 more from Amazon.
Try them, you'll be amazed!