Just realised I forgot to post the outcome!
It failed on the handbrake.
Assessment revealed a seized handbrake cable caused by water ingress at an outer cable break.
I stripped the brakes off Wed but I could see removing the old cable was going to be nasty. Access is difficult and I could see it was well seized at the hub end.
This morning it took me 2 hours to get it out. The lever end came out with moderate persuasion after gently bending the heat shield out the way.
There was no way the hub end was coming off by pulling it - even with mole grips. So I removed the support plate from the wheel side and after cutting off the eyelet and removing the cable I got access to the other side of the mounting collar and drifted it out easily.
Lunch time now. And then we'll put it all back together between showers this afternoon.
The outer cable break.
Support bracket after removal with eyelet cut off and placed back for illustration.
And the lever end needs someone moving the lever up and down to get a glancing alignment to get the thread barrel through.