Im an electrician, what I do is chop out the wall and the metal box to 25-27mm. Screw the Box to the wall, use 20mm round conduit and simply put the conduit inside the box so that it is approximatley 5mm inside of it. I then take to crampets (tapered metal hooks), which hold the conduit firmly in place. Job done and you can skim over the conduit, but you can usually without to much trouble pull new cables through if you want/need to.

Can be a pain going that deep in stone, but in brick and block it easy really.