I go cheap and cheerful, I use 'stardrops' available from Home bargains at £0.79 a bottle
I use it in a VAX carpet and upholstery cleaner, use hot tap water in the clean water tank and put about 50ml of stardrops per litre of hot water
I normally remove the seats from the car to allow better access (only 4 bolts on the A2 front seats and one connector), the colour of the water in the dirty tank shows what is coming out of the seats.
The VAX gets the seats dryish, on a summers day leave them outside in the sun for a few hours and they are bone dry, this time of year I would bring them into the house and leave a fan heater blowing warm air over them
They are not left at all sticky and come up like new, all stains are gone, I have cleaned a few sets of A2 cloth seats some of which were totally filthy. On really dirty seats allow about 3/4 hour per seat to get them spotless
One bottle of stardrops is more than enough to clean 4 filthy seats, I normally use a bottle and a half to clean 4 seats and the carpets and mats, and all for 79p a bottle
Cheers