Wait until warmer weather, and get a wet and dry vacuum cleaner. I have the grey and black one from Argos, and it's great for not much money. Then spray warm water onto the seats until the surface fabric is wet, and give them a good rub to loosen and dissolve the residue. Vacuum up as much of the water as possible, and leave to dry with the doors/windows open in the warm sun (remember that?).
You may need to repeat this a few times for really stubborn marks, or resort to very dilute APC (all-purpose cleaner, like Tesco's Daisy APC) solutions for dirt, or dilute white vinegar for scale.
You may need to repeat this a few times for really stubborn marks, or resort to very dilute APC (all-purpose cleaner, like Tesco's Daisy APC) solutions for dirt, or dilute white vinegar for scale.