I really don't think my issue is about excessive battery drain though.

At 13:00 yesterday it reckoned it had been active/on-standby for over 10.5 hours since the last full charge. That means according to the phone the last full charge had completed at around 02:30. Which is just wrong, it was on charge from 01:00 all the way through to 06:00 and was then back on the car charger from 06:30 until about 07:30.

So when I looked at 13:00 it should have been showing a total usage since the last charge of about 5.5 hours. So it looks to me that the real problem is to do with the phone getting confused and deciding not to recharge the battery even when it can.