On 25/03/2017 12:03, F wrote:
It looks as though I'm going to have to thread a thin, so very flexible,
cable into the ceiling void from below and then out again a couple of
metres further across. I've used a straightened coat hanger in similar
circumstances before but the current stock won't straighten to the
required distance.

I can't get at it from above, but at least the joists run in the right
direction for me.

So what's the best way to do it?

A good friend, a very good electrician, always uses the cover of a
smallish trunking for that purpose, e.g.
http://www.screwfix.com/p/tower-self...6mm-x-2m/64026

Flat, semi-rigid, and non-conductive, and he simply tapes the end of the
cable to it.