Replacing the springbase is not that hard. Take the seat out, then carefully remove the plastic side trims from the base. These get brittle and crack fairly easily, turn the seat over and see where they attach, usually a screw at the back and various push in hooks (I think replacements are usually available if you crack one).Disconnect any electrical connections to the backrest if you have seat heaters or electric recline etc. There are then two bolts per side attaching the backrest, take these out, rear ones first, and remove the backrest. I'm not sure exactly how the runners attach to the seat pan but its usually fairly obvious, look out for screws through the middle of the runner you have to slide them back or forward to line up a hole to get at it, and sometimes a spring for the slide release is also anchored to the rear screw, take carefull note of where it goes.
The seat cover has a cardboard edge strip rolled into a perimeter groove on the pan. Upsidedown squash the springs and get a screwdriver or hook under one end and peel it out. If there are little metal clips left in the groove lever them out and put them on the cardboard before refitting. The spring base then pulls away from the pad and cover.
Also
:-The pad will probably be cactus if a spring is right through.
:-Spring bases have subtle variations between models, years, spring layout, regular duty, heavy duty, updated etc, take your old one along when buying and compare closely.
:-I wouldnt try repairing one, difficult to get strong enough, and start again if unsuccesfull.

Regards

John