TD5 heater playing funny B******

The heater in my 2003 TD5 Discovery is playing silly b******. It is the climate control one - if I set the temperatures for say 22 they will sometimes blow really warm or sometimes it feels like the aircon is on! When this happens the only way to get warm air is to crank it up to 26 - 27. The engine temp gauge always reads normal.

Any ideas?

First thing to do is to check the coolant level and bleed well the system.