Moved house a few months ago and had a problem with a broken toilet flush valve. Whilst isolating the water, I noticed I have a pressure reducing valve. My static water pressure on the gauge is showing nearly 5.4 bar (78psi)...which seems quite high from what I read. When water flows from a tap, the pressure drops to about 2 bar (so the gauge appears to be working). I tried turning the nut on the top of the valve a couple of turns to the minus and it makes no difference to the gauge reading. The red needle doesn't move at all - what is this for?

I'm tempted to call a plumber to deal with a slightly leaky stopcock anyway. Should I be getting him to look at this too, or should I just leave it?