1998 Jaguar XJ Sports (V8 3200) Heater works erratically

The heating fan and display works fine, but the heater itself works only occasionally, regardless of engine temp or how long it's been running. Without touching the controls it can go from blowing warm to blowing cold, no matter what temp it's set at. I bought the car at the start of last summer and have no idea if the problem existed then. During the summer I had a major overheating problem as one of the rad fans had jammed, the system actually blew a hose off. That's been rectified and the engine temp behaves normally now - I mention it just in case it may have caused the heater problem. The coolant level falls very little, I top up maybe half a litre every 3 months (guess). The aircon works fine. The car mists up very easily in wet weather and I constantly have to switch on the demist fan which also switches on the aircon and blows cold air to clear the glass.

Any pointers would be most appreciated.