2.8QS Battery Issues

[Titan]

I'm hoping for some assistance with a battery issue on the A8 (actually belongs to my Mum) Apologies in advance for War and Peace but trying to get all the relevant info in.
Car wouldn't start on Sunday night so I put the battery on charge for a few hours (to let me Mum travel home) and tried it again. Car started fine and I put the meter across the terminals when the engine was running and lights were on and it read 14.0V.
(I didn't have heateed rear screen etc on, just the lights)
I decided to charge the battery properly once she was home, so followed her home and charged it on the bench.
The battery reading was 12.61V fully charged. After 24 hours on the kitchen floor with nothing connected, the reading had dropped to 12.25V.
I thought this looked like the battery and took it to Halfords as it is still under warranty (after giving another charge this afternoon to help with the test)

Halfords tested it with a Bosch BAT121 and it failed, telling me it was unserviceable and had failed due to a charging problem with the car so not guaranteed! "It is either over or under charging", said the assistant.
"I would imagine overcharging would be relevant since I'm standing in Halfords", I replied - No response........
The printout showed a test voltage of 12.38V, start capacity of 75%.
I asked if the battery should be fully charged for the test and he told me it was as it was over 12V........

Any help or advice greatly appreciated. I have VAG-COM lite for further testing.
If I have to buy a new battery then fine, it'll get my Mum back on the road. I suppose I will need to re-charge the battery and then test the alternator with a full load as well.
As an aside, my Mum is very hard on the battery as it's mainly local runs (2-3miles) and due to a bad knee, the electric seat/wheel is moved back and forward every time she gets in and out. Add on the heated rear screeen etc and I wonder if this is responsible? 2.5 years out of a Halfords 3 year warranty battery? If that's the case then fair enough.
Thanks in advance,
Grant