Replacing vintage electrical back box

[OldWood]

I am justifying this as Woodwork as wood is going to be involved somewhere in this

I'm rewiring a 1930's bungalow which must have had some re-wirng in the 1960's/70's (?). There's a mortared-in somewhat massive backbox for a 13A double socket, which took half a morning to extract as the box will not take a modern replacement socket. It's possible that two other sockets could be the same, but on the experience of this one I'll re-site those outlets.

I'm left with a hole in a half brick wall some 75dp x 100 x 150mm. The question is how to I fill this hole such that I can fit a modern 35mm backbox as I clearly cannot drill into the back to fix anything? Obviously a piece of wood (!!) to support the new box but how to get that to hold in the right position?
Rob

 

[Myfordman]

no nails, pink grip or similar to hold* a block about 40mm deep. once that has set hard, fit the box and feed wires in, then mortar, then polyfiller/plaster to finish.

* slightly over length thin off cuts (think chopsticks) to wedge across width of opening and press on block to stop it slumping.

 

[Jake]

I keep an end of bag of old plaster for this sort of thing, too old to use as plaster but it goes off like a shot. Wet the back of the hole to reduce suction, splodge of plaster behind the box, shove it in the hole until it's in the right place, prop the box with some wedges or whatever. When that goes off you have a new back of hole in the right place.

 

[XTX]

Spray foam and then a coat of filler over the top to finish it off.

 

[OldWood]

Many thanks guys for your help - I prefer to have the option of being able to screw the back box in so will go with the No Nails route and a block of wood to screw into. That's my reasoning but then I don't have any old plaster either !!
Rob