So, the saga continues!
I drove the car for 6 continuous hours and got a reading of 13 after switching off then switching just the ignition on which fell to 12.75 when I flashed the full beam.
All functionality had returned however the following morning I had to open the car with the key again, the battery had drained.
A journey of 400km followed by which time the battery was up to 13 again then the following morning the car was taken in by Listers of Coventry for the major service.
They reported to me that they had done an extensive test and the LHS battery had passed with flying colours and the only fault that came up on the scan was an intermittent fault with the power boot (trunk) which probably also explain the one-off closing phenomenon mentioned in the exchange of posts with Pierre.
So, next step is to book the car in here in France for exploratory work on why the battery is draining overnight even in warm temperatures.
Any ideas gentlemen?