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?

You could use drain rods, or a garden cane.

Bill