Go out and buy fabric conditioner. Decant it into a spray bottle with 80% water to 20% fabric conditioner. Saturate the car seat cover in it. Blow dry it with a hairdryer. The heat activates the surfactant in the fabric conditioner (alternatively do this with very hot water but it's best to use the hairdryer so you don't melt your spray bottle!)
When car seat is dry - vacuum it to remove any leftover dust.
Thanks for the tips, I think I'll start with the bicarb as I have some in my cupboard, then Ill try the others if necessary. I presume the bicarb won't damage the fabric in any way? Thanks you both again