Thanks gents
You hit the nail on the head. The main earth runs from the battery to the gearbox with a clamp half way along to give the chassis earth. Looks like corrosion here. I ran another earth from the body to the engine block, and now back up to 14.2v at tick over no load. Drops to 13.9v at tick over with every conceivable electric load on. Climbs back to 14v at a fast idle, so all looking like back to normal.
in incidentally I picked up on the varying voltage while driving because of the Sparkbright LED voltage monitor I had fitted, so thank you Simon As well!