I jumped the pigtail connections near the drier earlier and confirmed that the Low fan aux circuit came on (at least in 1 fan). It didn't sound/feel much different to the High fan test I repeated.

I let the car idle until it was showing somewhere over 110C (ambient 34C), but no aux fans cut in either high or low - with or without the A/C on.

The A/C seems OK but I'm going to get the refrig level checked anyway - probably Saturday.
I will check the Thermistor resistance values tomorrow as its mainly the High fan fail safe - or lack of thats bothering me.
I'll have a go at repairing/replacing the dodgy fan on the weekend.

Does that sound like a reasonable course of action? or any other suggestions?