Just come in from the Cold and Rain (having problems typing with numb fingers) after doing some checks and it would appear that the gauge is out.
Checked voltage across both batteries one was 12.72 and the other was 12.65 so both very healthy.
Ran up the outboard and checked the voltage using the depth sounder and came in at 14.4v at tickover, backed up by the meter on the batteries. Couldn't find the battery indicator function on the Garmin but to be honest I already had the info I needed. I then ran the outboard at 3000 RPM (woke up the neighbours
) and voltage increased to 14.6v's so very happy!
I'll have the gauge out at some point and measure it on the bench mounted powersupply I've got just to confirm the gauge is out of Cal and that there isn't any other funny thing that's happening. But I'm confident that all is a OK!