In article ,
"TMC" writes:

Although the outer leaf of the wall is brick the inner could be lots of
things e.g breezeblock (ok) or thermalite (very soft), the plaster could be
direct to the wall or dot and dab (therefore a gap between the plaster and
the blockwork behind

You may need to investigate a bit more

For thermalite type blocks, don't use a masonary bit or hammer action.
You want a clean cut parallel hole, for which they suggest using a
HSS bit (not masonary).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]