I've just taken delivery of a pair of new rear arch repair panels from www.beeversautopanels.co.uk
Not sure if antone knows of this company as i haven't been on the forum for some time. If not then they may be of interest to anyone with crumbling arches/sills etc as i thought these were all obsolete now.
They also list panels for 126/127/128/131 2&4 door/500/600 and newer models.