Can you test the overpressure valve by filling the CH loop until the
overpressure valve blows off: I
presume you have a pressure guage on your combi. Also, what is the
pressure of your boiler when
cold, and what does it climb to when it fires up the CH?

Operates at about 3bar, but "drips" virtually continously till pressure get
down to 0.

When refilled, usually take it to 1 bar, when operating (hot) may reach
1.5-2bar.