Thermostat light flashes 3 times when engine cold. Turn off and back on and doesn't come on again.why is this ?

Crud on the senser prongs in the header tank. Give them a scrape with a screwdriver to sort it.

Thanks. I'll.give it a try. Sorry to sound stupid but is that the circular tank where the coolant goes in? Pink coolant. 

There's one prong hanging down. It look yellow and quite fragile

Yes, there's usually two prongs in the coolant header tank to detect the coolant level. They get fured up causing the low coolant warning when cold. Usually a good scrape with a screw driver blade or similar will sort it. Not sure why it only tends to happen when cold and not warm?

Thanks for your help x