Hi I think I may be able to help. The problem is probably with the connector that fits into the the heater control module, because of its position down the side of the radiator it is prone to corrosion and eventual failure, I have just had my 2005 S Type 2.7D cured of this problem, the connector wires if badly corroded or broken will give these symptoms, its what I experienced. A replacement connector can be obtained by using the brake master cylinder connector of a scrap Ford Focus readily available & not in a position to be corroded, I had the new connector wires soldered on to the three wires after removing the connector from the harness wires on the Jag., result wonderful cold air on both sides of the system also hot as well if you require it