Thanks for the follow up replies.
I'm in the fortunate position of having quite a few spares - seatpost, saddle, bars, stem, wheels & cassette. Having swapped out the rear wheel and cassette together, for a longish ride this weekend and had no noise- I appear to have isolated it to the wheel or cassette.
Though I suppose it's possible that the through axle itself could have been the source - I usually torque that to 11nm so it shouldve been ok - but to make sure I could put the Elite rear back.
There's a short hill reasonably close to me, so I might make use of the extra hour in the evening to do some more tests this week