Cold heaters could just be trapped air. There's a bleed valve where the pipes connect, in the scuttle. The system should be filled with this open and the heaters set to hot.
Is the fan switch working? Check it in hot water. Does the fan work, have you tried it connected straight to the battery? Does the rad get warm at all? Perhaps the thermostat is stuck closed, hence fast warm up and no fan as the coolant in the rad stays cold and the switch stays off.
Have you checked the blue temp sender, or replaced it?
