Thanks for the quick response guys.
Its a breeze block under the plaster. On closer inspection it seems to be the voids in the blocks I've broken through to rather than the cavity.
Drilling into the sides might be a plan, also going to try a 25mm box.
I'm trying to install a new socket and am having some issues fitting the 35mm pattress.
The wall is an external cavity wall with the inner leaf being only 35mm including the plaster. So I have nowhere to mount the box to!
Unfortunately I have already knocked through a small area (which...