Nice one...
Mine recently started misbehaving and would only offer cool/slightly warm air when it felt like. Other times it would be rather hot...
Funny enough just today I had time to have a proper look at the whole system.
Started with the controls, removed them and all connections and pcb contacts are fine.
Then checked all hoses/pipes in the system(engine bay side really) and all is fine. Flushed some water from the Rad and started refilling, reving and blasting the heater as required.
It's now blowing pretty hot once again on demand... I will monitor it and see how she goes for a couple of days before I am 100% convinced it's sorted.
One thing I did notice was the Temp control unit, With the blower off(no noise), Temp controller knob when starting from full cold and moving over slowly to Hot(clockwise), you can hear the main units motor reacting to the changes, but only up to the 1 o'clock position then nothing.
Seems that it's only working up to about 20 degrees instead of the full range of 30. Or maybe thats how it is?