I had the rear wheel out to shim the rear suspension relay arm and whilst i was there I properly checked the play in the cush rubbers. Prompted by Pleiades view that 7,000 miles was about the cush drives limit.
Despite me saying a few days ago they seemed ok on closer inspection they're not and there seems to have been some contact between the hub and sprocket carrier.
The Wemoto pattern cush rubbers that were in there had done 10,000 miles and had a fair bit of play. I replaced them with the OE ones I took off earlier last year that had 5,000 miles on them (I also shimmed them with some old inner tube). They'll do temporarily.
Looks like I caught it in time - I'm off to Chemnitz this weekend on it too!