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.
So the battery is cream crackered although not surprising considering its open circuit voltage dropped to six! Although lead/acid batteries don't have a memory per se there is a scenario where they are regularly re-charged to only say 90% and they end up with 90% of 90% capacity and so forth in a downward spiral.
My own view is that a good brand such as Varta suggested by DaveW, will last for many years if treated well, but so will many others, again if treated well. Very expensive exotic brands may last longer still but in so doing leave you with marginal capacity for longer towards their inevitable and extended end of life.
"Treating well" means using regularly with an easy starting engine, and journeys long enough to fully re-charge. Not flattening whilst attempting to start a faulty engine. Being left permanently on smart charger/conditioner when not in use for more than a couple of weeks commensurate with re-cycling charger every two to three weeks if it doesn't do this automatically.
Pointless doing any further tests until you have a new replacement, but important to determine the level of static drain and reduce/eradicate once you have determined what's causing it. FWIW I would be tempted to stick with a new Halford's one whilst you sort the issue.
