Hello Alun,
If you ran the battery down just ‘playing with’ the electrics, then it’s quite likely the battery is well past its best, and no longer able to support a stop/start facility.
Yes, I understand the battery will be c just 4 year old ( and now out of warranty), but this vehicle is likely to have been caught up in the Covid issue of lack of ‘chips’ and might well have been sitting around for months - bad news for batteries.
If it were mine, I would drive it around to a Halfords store and get the battery efficiency checked, before doing anything else.
With respect to your wallet - again if it were mine- I would not be getting a new one at Halfords - just come back to us with the outcome.
If the battery is serviceable then we can move on.
Please let us know how you get on.
Regards,
Gareth.