Yep, I will get a new battery, but you miss the point about the BGB saying 15.1v. It has nothing to do with me, or my car or my meter. It is your car. Unwanted heat going into every component in the car, including the battery and that shortens the life of those components. I will also raise this with an auto-electrician to see what he says.
It MAY be a BGB error (wouldn't be the first). 

I'm sure you are right. Talking about it with a more experienced car lover, he suggested the high readings that I am seeing (14.8v, 14.9v) is because my battery is flagging, and that with a new battery, the numbers could well come down to low 14s.
 About the BGB saying 15.1v he was a little surprised, but suggested it is likely to be a very short-term condition.
I will still talk to an auto-electrician about this, for better understanding. I'll post here any salient points.

Good point - had not considered the voltage drop.
New question: (if I can insert a photo)....hmm asks for a URL, but my photo is on the C:drive...??
 
The braces that hold the battery in place use a longish j-bolt with a nut on top.
Unfortunately that bolt does not reach either of the 2 holes in the bodywork, behind the battery.
I suspect that something attaches to those holes and extends 20 or 30mm vertically, which the j-bolt then
hooks onto. I can't find a picture or part number to order it. 
 I should ask Josh if he has one.. or maybe just make something to fit....