Thanks all you sad bast**** with nothing better to do on a weekend. I appreciate it guys. Being an old retired engineer and general smartass who had been working with hydraulic valves and pipework before the Gas Safe wizard who checked my boiler was born ,I was pretty sure it was the PRV...
Got a slow depressurising leak. The drop from 1.5 bar to 0.5 took about 6 to 8 hours. Valliant engineer has found no leak on boiler and claims its in the pipework. I am not sure how he could detect a "no drop" in about half an hour when its so slow.
Trouble is its so slow that any water will...