I know I don't know BS7671 but I now find I can't even fathom the
current-carrying capacity if I put bog-standard 2.5mm T&E in plastic
conduit in an internal plaster/lathe wall and plaster over it. Is it
table 4D5A - despite the fact that methods 2 & 3 both involve insulated
walls which this ain't?

It matters because the derating for grouping would mean I could not put
2 cables through one conduit and hit the magic 20A figure - unless, of
course, I've misunderstood that too. So I was rather hoping for a
rating which took account of the greater conductivity of the wall and/or
the greater surface area of flat T&E compared to round multicore.

And, while I could revert to just clipping and plastering over, it seems
a shame not to use conduit (especially given my hindsight is now so much
better than my foresight).


--
Robin