In article , "Dave Plowman (News)"
says...
In article ,
Rob Morley wrote:
Make the hole too big (and too deep), whack in some Polyfilla, press the
box into position and support it with something until the filler sets.

Why not use quick set mortar? Quicker, cheaper and stronger.

Why not indeed - I tend to use whatever I have lying around (browning,
mortar ...). But if you're careful with the Polyfilla you won't need to
do any other finishing.