thanks chaps.
Not heard of S2CEB before.
The S2CEB, S2C4S6 CAT 6 stuff look perfect.
Thin, flexable and shielded for the mic line.
The last few rooms i built were wired with CAT6A for the audio lines, cheap and very easier to poke thru walls etc.
My own research found these as possibles.
Mogami W2789
Mogami 2814
and the Defcon 5 approach would be to wind some thin teflon hook up wire round LAV mic cable, inside braided sleeving!