On Mon, 13 May 2013 20:25:22 +0200, imanc wrote:

There's a lumpy mess and some cracked brick that is stil somewhat firm,
and the back box doesn't seat right.

Get one fixing ready, lob in some stiffly mixed old plaster, push in box
and use fixing. position and leave.

With one recess the back box is about 5mm too deep.

Is that a problem? You can get longer accessory screws if the ones you
have are too short.

I've got 3 more holes to chisel out so any advice would be appreciated!

I'd not pepper pot, just closely drill around the edge to a consistent
depth. It's easier to cut big bits out of a solid mass rather than one
full of holes.

--
Cheers
Dave.