Upholstery Cleaners - Lgd
My drivers side seatbelt has accumulated some ingrained muck that is leaving a mark across all my shirts when Ive been driving for more than a few minutes!

Ive tried a couple of upholstery cleaners from Halfords to try and improve things but to no avail.

Does anybody know of something that will do the trick?
Upholstery Cleaners - Aprilia
Trade factors will sell concentrated automotive upholstery cleaners in 5l containers - it not too expensive (less than £10 last time I bought some).

The last lot I had was made by 'NMC' (National Motor Chemicals??), made in Wolverhampton. Its good stuff and my wife uses it in the house too.
I think Machine Mart also see this stuff under their 'Clarke' brand - but it will likely cost more.
Upholstery Cleaners - PoloGirl
If it\'s a greasy layer of dirt, try a weak solution made up with soda crystals. (Just try it on a small, hidden area first).

Upholstery Cleaners - doctorchris
Try and borrow a steam cleaner which lifts the dirt but does not leave detergent behind to attract more dirt. The downside is it probably takes a day for the seat to fully dry out.
Upholstery Cleaners - OlafS
Try Auto Glym Tar Remoever. You can get a small bottle, along with car shampoo, polish, interior dash cleaner, glass polish and bumper care. If this doesn't work, the steam cleaner's your next option.