1.1 problem hellp quick please !!!!

[warrior]

my mates changed his head gasket on his 1.1 and every things all good besides the temp light on the dash is on. the coolant system is fully bled through and the thermo stat is opening. but when you start the car the fan starts up within 20 sec and then stays on after you have taken the key out for about 20min even though the water in the colant system is cold,

any ideas would be great.

thanks!