Cooling and heating problems

Hi all

I can see some of you here have had similar problems but cant find out about one.

I recently bought a 1.9tdi skoda octavia, seemed brilliant, noticed a week after the heating kept going cold, when driving with revs 1500 and up its warm to hot but when slowed down it goes cold, spoke to a friend who said the heater matrix is blocked and to put a flush in it, it has brought a bit of sludge out not alot though and the problem did seem to be getting better. A week or two later the heating stayed hot on my way to work when slowed down and stopped, I even sat with the car idling for 20 mins in the car park and still hot. Problem solved!!

That night when finishing work I started the car up and the water light was on, checked and it was down to the bottom of the expansion tank, topped up to max with antifreeze from work and went home, thought with the sudden loss I should check in the morning when it went cold, went to look and the coolant was upto the neck of the bottle. So did I actually loose coolant? Anyway took the top off and ran it for a little bit, level dropped to where it is supposed to be, but the heating problem is now back, also I have noticed it isnt losing water and there is very little pressure in the expansion bottle when top is removed, when I have time I'm going to do the timing belt and water pump, will flush out the heater matrix and radiator with hose pipe also for a while, and possibly replace thermostat. In the meantime any ideas on what else it may be