Rent a steam cleaner, worked well for me.
If you go to B&Q they have carpet shampoo machines in there for hire for £20 a day, they have an attachment for furniture too. Worked a treat on my car! Only thing is you'll need to air the car a tad for the rest of the day, as it'll be a touch damp afterward.
I'd be removing the seats from the car, too - both easier to wash, and let them dry.
Most seats will come out with just 4 bolts, and maybe a couple of electrical connectors.
Rent a steam cleaner, worked well for me.