Changed my head gasket on my 1.6 driver last week. Took it for a spin and it got quickly up to temp (half way on the gauge) and the heaters were blowing nice warm air for the firast time in ages! Came back and noticed I had a water leak from the plastic coolant hose manifold at the fron of the head. The one with the two coolant hoses, 3 electrical plugs that bolts to the head? Anyway i'd cracked it by having to chisel a rusty bolt off so down to the scrappy and got a new one. Swapped this over and this fixed the leak. Now the heater does not blow hot air and the temp only gets up to the firast notch on the gauge rather than half way even after an hour on the motor way!
Does this plastic manifold contain the thermostat? if so could this be causing the problem? I have the old switches which I took out of my old manifold so I could swap these over again.
As well as changing the manifold I also put some antifreeze in the coolant because up until then I had just used pure water to test for leeks etc, could this cause have caused the coolant to work more efficiently and thus not giving me any warm air? seems very unlikely!
Could it be the by-pass valves that everyone bangs on about? they are still there so may replace with copper pipe when I have the chance.
Any help appreiciated because I want my heaters back!

Acko