When putting in additional sockets behind the TV unit I used a multitool with curved carbide blade to good effect. I was installing sockets either side of the wall and wanted to limit the replastering to one side only. Behind the TV unit I needed three double sockets plate to plate in a row. The three back boxes were held in line by riveting some straps to the back before setting them in place. I was then able to carve out a neat hole with the multitool which did not require any filling admittedly the block work was the cinder type blocks and was quite easy to grind, scrape & chisel away. I also managed to do the same for a couple more separate back boxes.

Richard