So, after what seems like a very long time, I managed to get deeper behind the engine. Took the bonnet off (which I'd so light once the spare wheel is removed!). Removed air box to reveal the in and out coolant hoses from/ to the engine and through the bulhead to the funny aftermarket heater. They were past there best and a bit misshapen but not lose or split. I took them off to find that one of the through pipes to the bulkhead was very corroded, full of crumbly limescale type crud and powder. Dry as a bone too. See photo.
So I'm going to find some suitable hose and by pass the heater (as suggested) by connecting the input and output coolant connections on the back of the engine to see if that cures it. My only concern is that without a heater matrix it might still run hot.