Until you take them off you can't really make a judgement on the amount of work required, just have to see what's underneath there. Could just be crispy and brown in one place, could be all the way along
Unfortunately it turns out there was plenty of brown crispy stuff and holes hidden by the oversills. Currently looking at about £5k cost to replace outers/door steps and repair inner sills - a lot more than I had anticipated. May have to write the car off sadly or store it until such time that I have the facilities to do the work myself.