Update - for anyone interested.
I swapped back to charging from a USB cable via the cigarette lighter adapter and immediately the camera charges like expected. No further issue over having enough charge or apparently loosing charge.
Have now gone back to the fixed wire feed, but moved the piggy-back to another fuse. I was taking it from a 7.5A 'Acc' fuse and the camera was showing as charging, but I have now changed it to a 15A 'Aux' fuse. It also is switched rather than always on, but so far (since yesterday) I have not seen any loss of charge on the camera.
Despite what it says in the Nextbase user instructions, perhaps not all fuses are equal - at least on a Mazda.
I'll post a conclusion once I have done my regular short commute for a couple of days - a period which would have seen the camera out of charge.