Halfords should be able to test the big main battery, that will be the one to test. The aux battery is not really a battery, its a capacitor. I believe it hides under the footwell but could be in a different location from car to car. They won't be able to test the capacitor, it's a replacement item.
He done a recode for me which I don't think was previously done and mentioned there was a lot of system noise in the background so he had to wait a little while and he think if this doesn't sort it out there something that's draining the battery but they can't be sure about it.
Glad to hear battery is well on load but doesn't help if the problem is not cured after reset.
Is it working now currently working?
The capacity is also a component that can fail, I don't believe there is a way of testing this component manually. The item cost less than 1 hours diagnosis, so will be next thing to tend too.
I read on another thread about apps being left switched on. Try switching off the app, it may cause some drain to the battery