didnt say ya did cockjockey, just sayin gen renault quarters aint expensive at all.
n you can cut out the rusted section n weld in new sections but if some areas have gone, you might aswell just repalce the entire arch as ide put money the areas you dont cut out will start to rust anyway. can get universal arch strips from most car places for a tenner each. or get a genuine valver arch (which will probably still be on its way to rustyland) and weld that in instead.
either way, best solution is to weld in a new rear quarter both sides, might be slightly more expensive, but if you want it to last then your gonna have to spunk some cash. plus you aint gonna have 2 rear quarters completely clagged up with filler