Overheating ABH

It's started overheating in slow traffic, around town, stop start motoring.
The electric cooling fan is not starting on any speed, but does run if powered up by another source. I'm thinking of running a switched line as a stop gap.
Once moving at a decent pace, 50mph/80kph, it returns to a normal running temperature fairly quickly.

From a normal start, the car is warming up sensibly. Heater system appears to pass warm air as expected during warm up.

UK car. Was running fine, no gradual getting worse symptoms. It just got hot one day, on a journey I had done a hundred times.
Any suggestion regarding specific items, before I allocate a bunch of time to work out where all the circuits run?
Should it be possible to run the fan via the diagnostics cable?