Mk3 focus 1.6 tdci heaters not working properly

[Steallen]

Mk3 focus 1.6 tdci heaters not working properly and pressure in coolant system 

When the car gets hot heaters only go warm at best but if you rev it to about 3000 rpm they will go hot and when it goes back to idling they go warm or cold again also the expansion tank goes a lot lower after car has been running but when you open it water rushes back in this even happens when left overnight water is low in tank until you open cap then it rushes back in if I don't open tank water level stays low and doesn't rise until I open cap 

thanks

[drsdriver]

Possible head gasket problem or try replacing the coolant expansion bottle cap. The cap is a pressure cap, so this could be the cause. Cheapest option first. 

[TomsFocus]

Airlock in the system, just needs bleeding.