Join Date: Nov 2003
Car: MGF Trophy 160 in Trophy Yellow
The biggest cause of HGF on a K series is due to a coolant leak or lack of pressure in the cooling system.
This can be down to a pinhole in one of the hoses or steel pipes, a leaking water pump, a detached impellor on the pump, leaky rad (or heater matrix), or a faulty header tank /cap.
Normally if the thermostat sticks, it will stick open, resulting in overcooling (and a lack of heater). This normally won't cause HGF but will use more fuel.
The heater is on a bypass circuit, so water will circulate through the matrix whether the thermostat is open or closed. My Rovers generally had a nice warm cabin well before the thermostat opened.
HGF isn't as bad as all that. If the head is true, and the bolts are within spec, it's not too hard a job to DIY (or overly expensive). It is also worth upgrading to the MLS gasket and stiffened oil rail at the same time. Get a Haynes Manual and follow the instructions meticulously and you should be able to change the gasket easily.