Ferroli 701 losing pressure and leaking?

Hi, I wonder if there is a simple solution to the following problem:

The Ferroli Optima 701 loses pressure when the heating is turned off for any length of time (eg. overnight, or during the day), or when hot water isn't being drawn. Usually the pressure drops off entirely to 0 bar. Sometimes, but not always, there is also an accumulation of water under the boiler.

Previously it worked fine if water was let into the system. However, this morning when I tried to top the water up, it started leaking a lot. I've taken the cover off, and the water seems to be coming out of the top of the automatic air vent unit.

Can anyone advise, please?

thanks
Jon