Can anyone advise what depth backbox I should use? I'm adding a 2G MK
socket, and was planning on using 25mm - however after reading the
instructions it said that for = 9mm face plates, use 25mm, and 9mm use
35mm. I think the 2G MK socket is about 9mm - but I just wondered what the
benefit was with going with 35mm, and whether 25mm is very cramped (2x 2.5mm
T&E). Its an internal block + plaster wall (1930's block), so I don't want
to go too deep (not to mention that the deeper I go, the more work it is!).

Is 25mm a bit shallow?

One final thing - when I fitted some before, I set them too deep as I
thought they should be plastered into place. I think they should be screwed
into place and made good with plaster (rather than 1/2 - 1" of plaster being
the thing to hold it in place) - am I correct?

Thanks

David