PRV failure on Biasi Combi?

Had a Biasi for 4 years now and it's been a bit noisy but OK.
Recently been losing around a bar of pressure per day and this morning was at 0. On trying to increase pressure using water inlet feed it rose to 2 then just fell back to 0 in seconds. Asked my wife to do same while I checked the water release pipe outside the house and its just pouring out. Definitely not a leak in the CHS. Does this sound like a PRV failure or something worse?

Not intending to DIY unless its a v straightforward job.