Saunier Duval F35E auto-vents down to code 21 in minutes?

7 year old boiler, noticed code 21 (radiator circuit pressure low) so she opened the filling loop but left it open too long. It auto-vented, but kept going until the red light came on and code 21 again, in minutes.

Every time hot water is needed, pressurise it up to about 2.0 bar, then it will run happily until it depressurises.

(Since the water supply doesn't depend on the radiator circuit being full, why does the whole thing shut down??)

Suggestion (from a gas engineer pal) is the 3 bar valve has stuck open, which I am confident I can replace, but I would be grateful for any other opinions and/or advice.

Many thanks