heres one of my rear arches after a clean up, mark on here where yours is rotten so we can see where you mean exactly.



lower rear inner arch rot



the lower front inner arch rot



rot nearly all cut out





the nearly finished article



cant find the pics of the underside after welding and plating, will have a good look later, see if they turn up

its not too big a job for a bodyshop to be honest, pop down to your local place and show them, see what you're looking at paying.