how can i sort out my cooling system 2002 ford galaxy 1.9 tdi?

I have an 02 ford galaxy,the temp will be fine (half way mark) but gets warm the longer you drive.The thing is the main header tank looks empty when you check it after driving and when you go to it the next day to fill it,take the cap of and it refills itself to overflowing. had sniff test-ok,checked rad-ok,checked return pipe and flows,the only thing we think it could be is an airlock in the system near the aux heater at the back of the car.been to for and 'useless'thought i was mad,until i explained the problem.and they said they never normally check for that so could it cure the prob if we bypass the aux heater and join the pipes together......Also can the aux water pump cause the problem...the cooling system is always pressurised and have to open cap ever single day before driving