Light switch back box help

Surface mount back box or sunken backbox?

What is the wall construction, plastered brick or plastered blocks, lathe and plaster or skimmed plasterboard?

If the box is sunk into the wall

Expanding foam.

Plaster - paint the hole with PVA:water 1:1 and allow to dry to stabilise cheesified surface and then "glue" the backbox in with plaster behind it and at the sides.

If it's a surface mount backbox just hack out the wall behind it, again seal the surface with PVA:water and let it dry and repair with filler. You may/will have to hack out where the old screws went to the depth of the screw and to a diameter of perhaps half an inch.

Open out the hole sufficiently to allow you to fit a piece of wood at a depth that the back box can sit on. PVA all the old brick/block/plaster. Bed the wood in bonding plaster. Attach the back box to the wood and fill all the surrounding with bonding plaster. Skim/fill/sand as appropriate. Alternatively, it's sometimes possible to fix sideways into the wall ...

Expanded polystyrene

Revisited the problem todau.

As luck would have it, while there was a brick back, the actual wall has some wooden panelling. I'd forgotten as it was papered over within a millisecond or two of moving in.

So I was able to use a drywall backbox to sort it.

Of course it took about 30 tries to remove enough of the back brickwork so it would fit flush. What with me only having a hammer and chisel.

As if to taunt me, the next one on my list took just over 5 minutes to fit. And that includes finding a rawl plug to fill with sealant to insulate off the mystery 3rd wire. (I think it's a hangover when the light was 2-switch operated like a landing. But since we've never found the other switch in 22 years, I can live with that.

Tomorrow the last one - a 2 gang smart switch.

All of this made piss easy by the fact all our bulbs/tubes are LED. So no fiddly capacitors needed across the circuit.

While not applicable for this one off, and you already had a chisel, a scutch chisel and comb makes jobs like this easier if done by hand.