I need to put a ring main in a small extension to the main house
(currently it has no power). The original outside wall to the house is
a cavity wall, and the flat roof of the extension meets the house wall
below the level of the ground floor ceiling. Floor in both main house
and extension are solid, and all the wiring is above the ceiling therefore.

So how should I get the cables across the cavity and into the extension?
There is no way through other than through the cavity. Should I pass it
through a conduit of some sort to protect it within the cavity? Any
thoughts (including what the regs say on this) much appreciated.

Thanks
David