Just in case someone happens across this post, the solution was in the end quite simple, however the indicators are not so obvious.
I fixed this issue by simply replacing the battery. It is important to ensure you have a slightly more sophisticated diagnostic tool capable of completing the battery registration. The consequences of omitting this can cause the car to overcharge the new battery and shorten its life.
It is reasonably well documented online that if weird and unexplained dashboard lights or false triggered DTCs appear with scan tools that it may be the battery. What threw me intially was the fact the car started fine, there were no dashboard fault lights and just some random DTC faults, which if cleared didn't re-trigger.
I had other cars to drive, so got a big lazy but in the end I purchased an Ancel VD700. This has the EPB piston retraction function, battery registration and other useful stuff.
The battery test shows the current voltage and importantly the lowest voltage during an engine start. In my instance, it dropped to 6.7v during startup, which was the biggest indicator yet that it was indeed the battery.
Check your battery spec carefully, physical size, terminals, Ah rating etc. It's fine to put a 95Ah in a car with a 92Ah, the registration software will ask for this value and the car will charge it accordingly. Mine is Start/Stop so AGM, this is important.
Fitting is ok, they are heavy (15Kg). Make sure you remove the -ve terminal first then the +ve. Wrap the terminals in a cloth when changing the battery. When replacing terminals, +ve first then -ve.
Battery registration is easy, follow the prompts on your chosen scanner.
I repeated the battery test and the lowest voltage was 10v, which is what I would expect is nominal.
After a test drive, the Stop/Start works fine and to my relief the infernal bleep had stopped.
I did read somewhere, there is a battery low warning, but that did no manifest itself on my car.
When you are used to car batteries either being good or flat, this liminal state can throw your own diagnostics off track. Hope this helps.