Thanks - I thought that if the vessel had failed then the pressure would shoot up over 3 bar and water would be ejected? I'm pretty sure there no rad full of air to absorb the increase either.

I checked the pressure vessel with the water pressure out of the system and it's 9psi - the manual says it's set at 0.5 bar initially although there's a table (9) in the manual that doesn't makes sense to me (I can't tell which is the pressure vessel setting and which is the water presure setting).