pressure loss in my Vaillant boiler?

my vaillant boiler model VC GB 242 EH is installed as a pressurised central heating & hot water system.

The pressure was set to 1.4 Bar when last serviced.
Suddenly, water is coming out at the overflow outside.
The pressure then dropped to 0.2 approximately.
When topped to 1.4 bar again, the same story repeats.

I suspect the internal Expanson Vessel. Do you agree?
If you do agree, if you have a picture to show where the repressurising valve is located, I would be grateful.

There are no leaks visible inside the house anywhere.

At this low pressure the CH is working as before! But the fault whatever it is must be rectified.

Thanks

Amal Roy
Peterborough