my dads cosworth keeps overheating due to the radaitor fan not coming on at all.

the only thing i am gettin is that it only happens after an hour sat on tickover and it goes into the red, then i get the header tank and it bubbles over and dissapears thorugh the bottom off the car.

i have already tried the themostat and put a new one in...

any ideas or when does your fan kick in? help as this bits stopping me from driving it