Update on the electric fan situation.
Whilst brother-in-law was changing a headlight bulb, he noticed that there was a load of corrosion around the thermostat housing. When he gave it a poke, he saw that it was breaking up. So a new thermostat unit was ordered, and he has now fitted it, and all is back to normal.
A complete thermostat unit from ebay was a grand total of £27.00. So a good cheap fix.