Best way to fit a sunken backbox into a breeze block wall?

If you are careful you can use a 3/4 " 20mm wood chisel you will never cut wood with it again but for 5 holes it will work better than any cold chisel or bolster chisel you can use a 2lb club or lump hammer but take it very steady the art is to chip away at a very shallow angle to the wall and not break through into the hollow section of the block try and position the boxes close to the side end of a block and cut half in one block and the rest in the next Block this area of the blocks is solid and you should be ok

Realistically,your better to do it by hand with a hammer and chisel.It may also help if you have an angle grinder to cut out the outline and reduce damage.

The best tool for the job is an electrical box sinker. This does require an SDS drill, but you should be able to hire both relatively cheaply for half a day, or in fact 10 minutes in your case (it takes approx. 2 mins per box)!

use a small grinder and a sharp chisel