COMBIcompact Vaillant boiler water pressure problem

[H4YES]

COMBIcompact Vaillant boiler VCW-GB 242EH or 248EH problem: The pressure meter is on 3 (should be on 1 when cold) and the overflow pipe drips profusely, presumed because cold water keeps coming in, so I tightened up the cold water pressure valve (?) as much as possible and the white plastic cover is now broken. Despite this, the problem seems to be getting worse, with the pressure increasing noticeably whenever I empty the boiler via the overflow.
I am no good at DIY. Can anybody help please?

 

[Gasguru]

The filling loops still open/failed see FAQ's.

Or perhaps if the boiler still has the plastic secondary heat exchnger it may have failed.

 

[H4YES]

Looking at the illustration in the FAQs, it was the filling loop I was tightening up, not cold water pressure valve. I could not locate any stopcock that would stop the pressure rising, with corresponding leakage from the overflow pipe, so had to close the water supply. Could anybody help please? I can be contacted at schran*ka2004 at vo*lny dot cz (remove the *).
Can I run the central heating with the water supply shut off and taps dry?