You can run cables buried in plaster etc. at any depth as long as they
go vertically or horizontally from a wiring accessory.
If for some bizarre reason you wanted to route differently in the
middle of a wall then you must either protect the cable with a serious
protecting plate (grounded) and not thin trunk, or bury at 50mm
You can run cables in a band up to 150mm out from the top of a wall or
from a corner, but not from a floor.

Great, thanks for that - the OSG was out at the library :-)
--
Dorothy Bradbury