Dear old Dads problem is not with his actual aircon. What you describe is a common problem with 75s and I have been there. The main radiator fan unit has packed up.When the car is in motion the airflow passes over the unit and is cooled. Nothing happens when it's stationary. Open bonnet and switch aircon off and on. If the fan is working it will cut in and switch off each time, to be sure do it when the engine is cold. If it's a petrol engine get the fan fixed asap, but I get the feeling it's diesel. Mine went 18 mths. until I discovered what was wrong.New unit c.�300 from a supplier, not a Rover dealer in N. Ireland. Hope this helps.