DON'T use thinners, this is only really any use when people are trying to resurect brocade trim that has been painted. It doesn't do the vinyl any good at all.
If you are just cleaning it, then as has been suggested above, flash or similar will do the trick, stick to "normal" cleaning products if the grime isn't too bad, that & lots of scrubbing.
If you have any specific stains that won't go, as about them specifically, paint, rust etc will all come off more or less OK, but the treatments are not recommended for full panels & need to be done carefully.
I used a bar of Vanish soap on the trim and seats in my MK1, probably similar stuff to Flash. Just make sure you wipe them dry afterwards as the soap can leave rings where the water dries off, probably dues to bleach in the soap.
Thanks for the replies. I tried some vanish (as I had some in the cupboard) and its done a pretty good job! I'll see what they look like fitted, as they may need one last scrub.