On 31/08/2016 08:18, dennis@home wrote:

fit a spare box in the hole, fill around it and remove the spare box.

That's not a bad idea.
Didn't get into screwfix to pick up the plastic extension bit yet.
I was thinking about cutting a spare box into rectangular sections to
leave in as permanent "wall" to give the filler a solid backing but each
section might just turn into 4 individual bits.