could you take the pipe work off after the stop cock and put a pressure gauge on so you have a starting point and make sure the pressure is ok, then you could put the pressure gauge on the pipe work after the stopcock and see the difference, it gives you something to work from.
IF you had a clue what you're talking about you'd know there's a 1/4" BSP socket on most of these 22mm Pressure Reducing Valves, IF the OP could get access, he could simply connect a standard pressure gauge & test it. Me, I'd just simply replace the PRV.