High Pressure Problem on Sime Friendly Format 80E Combi

[macman81]

I have one of the above combi boilers and recently it has started to run at a higher pressure than before. It used to be about 1.5 bar idle and 2.5 bar running, now it is reading about 2.5 bar idle and 3.5 bar running (maximum on gauge is 4 bar). I've checked the filling loop valve and it appears to be fully shut, have turned it on and off to check operation. All OK but some signs of limescale around the valve, though not visibly leaking. Could it just be some dirt in the inlet valve stopping it closing fully (I live in a hard water area) or could there be another cause? The boiler is otherwise running normally.

 

[bengasman]

disconnect the filling loop, lower pressure and see what happens

 

[macman81]

I've had another look and tried manually operating the pressure relief valve. It dropped to below 3 bar (as it should), but then repressurised itself back up to 3 bar and is now filling and venting continuously to hold at 3 bar, even with the filling loop valve supposedly shut.
So I'm guessing that this is a combination of a sticking PRV and a leaking filling loop. I'm having both changed tomorrow, but if this doesn't cure it, what else should be checked please-I think it could only be a leak in the expansion vessel?