Richard,

I tend to drill the securing holes & plug before the dollop of plaster -
as
you tighten the screws pulling the box home it squeezes the plaster out &
it
can be scraped off flush. All rock solid & secure once it's set.

That makes sense, thanks; though I do recall on some past attempts that the
brickwork had crumbled so much that drilling a hole before putting in the
mortar was simply impossible. Probably I've just been unlucky with my
walls. I take the point that it can be damaging to try to remove the box
once the plaster's in and set, though.