After some wiring testing at the alternator light, Kevin came to the conclusion that it must be the regulator in the alternator causing the problem.. So it was taken off and taken to an alternator specialist for testing. It wouldn't be ready for a couple of days.
The good news is that we have been OFF shoreline now for three days and the batteries are still at 80% (26.4v) in the morning, and the solar panels charge us back up to 100% (27.2v) by midday, so we haven't needed our engine to charge the batteries.. So they are right, Lithium batteries are great if you can sort out the charging!!!
Chuffed with our charged batteries we watched the football and when we went to bed the readings were back at 80%, but what was weird was that the solar controller had turned off and we didn't know why! We are still learning!