Lawrence Zarb wrote:
It is a 9in aerated dwarf conservatory wall

set up a dust containment zone with plastic dustsheets and gaffer tape.

mark out the area to be cut and cut the perimeter to 50mm depth
using a diamond disc on a 4" angle grinder.

cut parallel lines 50mm apart, 50mm deep between the external lines

chase out with an old 50mm chisel.

there are other methods.