By way of an update, I took the car back to the dealer who supplied it (independent) and they had their technician take the battery out and charge it for 72h, presumably with a charger bigger than my CTEK MXS 5.0. As soon as the battery was put back in, it fired-up as normal, there were no codes showing and the roof and service flap opened as intended. He had to take it to the local OCP to have the driver cards supplied and coded, so I asked him to have them plug it in and run a diagnostic. He moaned that they were going to charge him for that but soon came round to my way of thinking. So last Friday, I picked it up from the local OCP and they confirmed that no errors were showing and the car was in perfect health.
Now, when I say "local", I mean local to the supplying dealer - it's a four hour drive from where I live! So I decided to drive south and was able to meet some friends half way. I stopped the car to take a schedule business call (outside of the car, it was switched off) got back in to drive off and the battery is dead, once again. The very same signs, the car would not start, it just made a ticking noise and every error under the sun was thrown-up on the display. I was pretty annoyed and called the dealer, who was equally annoyed but said it sounded most like a battery so he ordered me a new one from another OCP near to where I was. One of my friends came along and we jump-started the car, after first going through the hilarious frunk opening process, and it once again fired-up but with no codes, the roof opened etc.
Now I have a brand new battery fitted and have also installed a CTEK Battery Sense device. Interestingly, when I got back from my long drive, the battery was only showing 80% and so I put it on a CTEK tender and got it up to 100%, where it has stayed. I've taken the tender off to see if I can learn more about the profile of the battery for the next couple of weeks. At the same time, I've bought a second-hand CTEK MXS 25 from eBay and intend to use that in future to make sure the battery is always in good shape.
My suspicion is that in the previous year to my owning the car, where it had only done 800 miles or so, the battery had been deep discharged on more than one occasion to the point of ruining it in some way. I'm not a battery technician so can't postulate on what might happen to an AGM battery, but I'm sure you all have your favourite layperson's term you can use for something that no longer works as it should! So it would take a charge, look as though it was fully charged, work for a bit and then simply fail. Maybe someone on here has similar experience with Porsche or other 12V device. Either way, it appears to be working and now the service flap works, I've been able to replace the engine cover with the silly titanium painted version and also the aluminium effect radiator and oil caps. Because why not?