It looks like it could indeed be the cooling fan.
I decided to check the water pump was working ok and it's pumping nice and freely with a consistent stream.
Had a general nose around the fan and spotted that there was no cover over the electronics on the back of the fan, looking a bit further, the cover was lying at the bottom of the engine next to the flywheel pulley! Goodness knows how that's happened, think I'll replace the fan first of all.
See image below, weird.
Hopefully that will be the fix? Just need to work out how to remove the fan housing now
20230510_185333_resized.jpg