Update:
I disconnected the battery, so no connection to the car. After 36 hours in "Recovery Mode" on the Ring Charger It will not go above 60% charge.
So my conclusion is that the battery has developed a fault.
Back to Halfords. The only battery they stock that fits the Plus 8 is the one I have: there are no other options. I paid £71.00 in April 2019, it was replaced for the same reason a year ago.
Third time lucky?
Or should I cut my losses and get a substitute: because of the way the battery is fitted in the Aero Plus 8 there are limitations on size, in particular the Optima Red Top is too long: the maximum is 242mm, the red top is 254. The best candidate seems to be an Odyssey model.
