Socket box problem?

Depends how the thread is knackered, but you could try this:

Another thing. If it's a really old box, the thread may be different. Are you using one old, and one new, screw?

I've taped a nut on the back of the lug before now

restoring tool (tap) into the hole.

Thanks Ive just phoned the electrical wholesaler and they sell 3.5 and 4mm hole restorers. I presume its 3.5mm?

If it's an old box, chances are that the threads will be 4BA - which is a similar diameter (about 3.6mm) to the newer 3.5mm screws, but with a different screw pitch. If you mangle the lug a bit (as suggested by another poster) you'll probably be able to tap a new 3.5mm thread in it, using a hole restorer.

The alternative is to use a slightly larger (provided it will go through the hole in the fitting) self-tapping screw.

Use a 4.0mm rethreader and a 4.0mm bolt if the threads are really knackered.

Adam

Drill out the remains of the thread and superglue a nut on the back.

Since they are all galvanized you could probably solder a brass nut on too, given a decent sized iron. Not actually tried it, but I have a similar application which I need to fix some time.

I think that if it's virtually impossible to replace the whole box, the best approach would be to attach a 3M5 nut to the back of the remaining lug using Araldite, holding the nut in place with a screw well-greased to stop it being 'Araldited' in place.

Alternatively consider Helicoils :-)

It really isn't all that difficult to replace an entire box if you score around it with a Fgnayrl knife, avoiding the cables; there should be a minimal amount of repair plastering to do.