Why does your 2003 Rover 75 Connoisseur air con produce heat only intermittently?

There is no vehicle called a "2003 Rover 75 Connoisseur." The Rover 75 was not produced after 2005 under MG Rover, and the name "Connoisseur" was used with Rover vehicles prior to that. However, it is impossible to definitively diagnose a problem without inspecting the vehicle itself. Possible common factors can include refrigerant leaks, condenser/radiator fan assembly defects, or control/sensor switch related faults.