After a saunter round B&Q looking for anything that vaguely resembled conduit I ended up buying two lengths of the 110mm x 54mm x 1000mm conduit shown below and four of the elbows.
Enabled by the fact that the majority of the length of each run is both in a fireplace void and behind a sheet of plasterboard that I can fit after fettling with holes and brackets, I should end up with the very discrete solution that I wanted; white to match the walls, no visible lugs or screws, and not even a faceplate if I am careful enough to get the exposed ends flush with the wall and plastered-in accurately.
I might even cut one end of each elbow back so that it protrudes by exactly the amount to make it neatly flush with the visible plasterboard face and devoid of cutting "whiskers"; this will reduce the effect of the elbow and make cable drawing even easier.
Thanks for all the help and ideas, and I certainly now have some pages bookmarked for other projects I am planning in the future.
For anyone else considering a project like my current one, the product I purchased is Manrose low profile extractor fan ducting.
Cheers for now,
TLXV