I have read other threads on this topic but my problem seems different. I replaced a heater core yesterday and since then (a) I have an 'X' on the CC digital screen. and (2), every time I turn the car off, the mode defaults back to demister. There is a short delay before the fan cuts in after activation as well.

From what I read, these type of problems are often caused by vacuum leaks.

The car is a Series 1, VX Berlina.