UPDATE:

I took the car to have the battery tested today.

The tester tested the alternator and that was fine.

He said the battery is in very good condition and the cranking amps are high, but the voltage was still only 12.4v.

He said something must be draining the battery, so he attached a gadget to the battery positive cable and we closed and locked the car and waited 16 minutes for it to go into full sleep mode. At this point he read the draw and it was only 40ma. So it appears there is no parasitic draw when the car is locked and asleep overnight.

So apparently the battery is good and there is nothing drawing from it when it's left locked!

He could not understand why the battery is only at 12.4v when it should be at least 12.5v (preferably 12.6v) when the battery is good and the draw is low when the car is "asleep".

I wondered if the battery fitted is not heavy duty enough as it's rated at 88ah and 750amps as their system recommended a battery with 100ah and 900amps. But this seems unlikely as I have read the OEM battery is only 90ah and 720amps.

Can anyone shed any light on this please?