So I've got some answers, so posting in case it helps others...
@katchaplin had the perfect answer, it was the mapping of mixture.
There is freeware scenery for RAF Valley (EGOV) and there appear to be differences in the terrain levels.
Once again a mapping conflict. I searched for the landing light and found a button mapping that was in conflict with and switch on the cockpit panel.
Yet again a mapping conflict... starting to see a bit of a pattern here. I had the Autopilot Master mapped to a switch and that was stopping / conflicting with the EFB buttons.