My drivers seat is really soft and giving me crippling back ache.

I'm planning to replace it with a new foam from a base model car as I prefer a firmer place to park my bum (apparently the higher end, non-sporty models had softer seats for "comfort")

Anyone got any idea about how easy it will be to change the cover over? I know it's hooked down in lots of places, any tips?

Cheers