Vaillant CombiCompact VCWGB221H (GC.47-044-14)

[Vic1]

Hi
Please can you help me.
I have inherited a Vaillant CombiCompact VCWGB221H (GC.47-044-14) - I am not sure how old it is as I just moved into the house.
It plays up a bit in terms of firing etc but now i have a serious problem.
It is heating water up no problem, but the central heating won't switch on.
More specifically, it fires up for a couple of seconds, then dies away till just the pilot light is on. In the meantime it is making really loud watery noises - much like when you have flushed the loo and the water is filling back up. The only time the noise stops is when it is heating the water up for the hot tap.
Any suggestions?
Thank you
p.s. I am freezing

 

[kevplumb]

whats the pressure gauge reading

 

[Vic1]

it reads just over 1 - which i think is OK

 

[kevplumb]

check the servo valve is returning properley first

if its not the diverter ain't either