Get the pressure washer on em, take the seats out and blast em with water, dont put the lance to close cos u will damage the seats. then leave to dry in a garage/shed/kitchen next to a storage heater/dehumidifier(spelling??) this could take a couple of days to dry but well worth it as the amount of brown murky water that pours out is great. plus your cleaning all the foam and not just the top inch that u would with normal cleaning products.
or u could go spend £35 and hire a carpet cleaner for a day but again your only getting the surface crap out