Problems with the heating in TD5

I'm having problems with my 2002 110 TD5. The heating inside the car has a life of it's own. Sometimes it works fine and then suddenly only cold air is coming out. The needle in temperature gauge is a little to the left of 12 o clock. If I turn on the Webasto while driving when cold air is coming out I can mostly get hot air again. When the Webasto has run for 5 min I turn if off and hot air will continue, sometimes cold air will return. It is different when the hot air stops, it can be in city traffic or on the highway.

The Webasto runs every morning and the car delightfully warm when I get in.

I've noticed that there is some overpressure in the system next morning (before the Webasto) if I remove the cap from the expansion tank. I can also see traces of coolant on the expansion tank.

I'm loosing about ½ litre of coolant every other week. I can't really see any leaks in the system.

Any ideas of where to look?

2002 is the change over year for the td5 engine, they went from plastic head dowels to steel head dowels to stop the cylinder head shifting sideways, loads of other changes as well.
I would say you have a gasket failure on your hands.