Vaillant VCW GB242 high pressure ?

I have a VAILLANT - VCW GB 242 EH boiler installed by the previous previous owner and no manual.
When the heating comes on the pressure goes up to as high as three bar. I have read one cause can be insufficient air pressure in the expansion vessel. Can anyone point me in its direction please and can you confirm that is needs to be set at 0.65 bar (10psi). I am supposing it is internal but have only found the air intake at the rear on the RHS