Ok BIG NOOBY alert despite being an owner for 7 years
So when I tested the 1st and 2nd stage fan settings as per Mocki's instructions THIS TIME I KEPT THE BATTERY SHUTOFF ON and of course both stages work perfectly fine. Vanhypochondriate symptoms...
So, fan works properly as it should - we tested this past week with a 899 mile NC500 wild camping road trip and had absolutely no issues with the coolant and radiator. Mind you I did put a bottle of radiator gunk in to seal the weep and that worked fine as well.
So, now I'll replace the knackered rad this month and renew a few hoses and stop worrying about it.