Does the engine reach its normal working temperature after 10 minutes of driving? If not then suspect the thermostat. If it does then you need to check the two pipes that go through the bulk head (to the heater matrix). If both are cold then suspect the heater control valve. If one is hot and the other cold then suspect that the heater matrix might be blocked.
If both pipes are hot then would suspect that the heater flaps are either stuck or have a electrical fault.