Anyway I didn't get far, since I realised after 6 1/2 miles that I had forgotten my carefully wrapped up smart clothes, into which I was due to change! Doh!
On the way back to the house, with some renew urgency, since I would now need to do a quick turnaround, I discovered that the performance of my Shimano BR-CX77 cable operated rear discs was non existent. No fault of the mechanism as far as I can as yet tell, but definitely a problem with the adjustment.
I hadn't touched them since I came home last summer. Principally because I'd never looked at how to do this before. With plenty of other seemingly more pressing distractions, I had ignored it to the point of near failure.
Lesson 1. Don't ignore stuff, especially when it's your brakes!
So this morning I have downloaded the Dealer's Manual from Shimano
Good to have learnt how to do this at home and not half way across the Alps.
Miles for the day 13
Postscript. The pads come out easy as and are very worn, particularly on one side. Since I was unable to identify the right ones anywhere else, I have ordered 4 sets from Canyon and will have to content myself with planning or riding with poor rear brakes, while I wait for them to arrive.