working on daugthers old property - back boxes are sunk below previous owners re-plaster so cannot remove/replace - screw threads are not metal (some kind of fibre material) and have stripped - any ideas how to replace/repair/etc???

I have in the past used a 4mm rethreading tool and replaced the 3.5mm screw with a 4mm screw.

But in the main it's a bax replacement.