I took a reading as you suggested, and it shows 12.1V.Personally, I would take a reading at the charge point under the bonnet. You would be looking at 12.6V or above. The batteries are wired in parallel so if you measure at the batteries and they are different you've got an even more interesting problem ;-)
Is this too low and is it this that is causing my first key turn "donk" first thing in the morning.
I am going a long run tomorrow (300 miles or so) so presumably that will charge the battery back to 12.6V.
What do you think ?