Please sanity check my charging issue thought process here:

[Ted]

It definitely doesn’t sound like a battery issue however a load test on the battery would certainly clear up any doubt.
Lots of things that it could be including a slipping drive belt, but as the alternator has been changed…
I’m with @E55BOF here. Get a good auto electrician to take a look and see what they say.

 

[MarkCorrigansBoiler]

Thanks again for all your input.

I take the point about the battery, I probably should have started there, but as I'm £350 in already, I'm reluctant to chuck more money on speculation.

The latest update is that the mechanic has checked and cleaned all the earths, this didn't fix it, so he replaced the alternator again and ran it for 40 minutes, and so far all is good. I also drove it home for a couple of miles which was fine.

I'll find out for sure later as I have a 90 minute drive in the wet with wipers, lights and blowers on full. If it survives that, I'm hopeful that it was just bad luck with the first replacement alternator (it was a remanufactured Bosch one).

Cross your fingers for me folks, and thanks again.

 

[MarkCorrigansBoiler]

About 60 miles done and the issue seems to be sorted. Rock solid 14-14.1 when cruising, occasionally dropping to about 13.6v when idling with lights, wipers etc. on.