thermostat housing rover 75 04?

hi i have rover 75 year 04 i have bottom hose stays cold car does not over heat heater hot is there a problem with the thermostat at all if so where is the housing after a hour running the temp gauge goes off and if i put the car fan on the screen the fan for the rad comes on if i put the fan onthe intireor the rad fan goes off if its a problem with the stat could someone tell me where the housing is thanks yours paul,