I'm installing an irrigation system for some trees planted a few months
ago, a three hour trip to water them every few days is getting a bit old.

Water supply is 25mm blue mdpe terminating in JG speedfit stop tap and
3/4" tap connector, I suppose the pressure is "highish" because when I
reduce it down to 1/2" tap connector it has a tendency to blow the
hozelock connector off a hose.

For the irrigation pipe, I've fitted a pressure reducer that can take
input of 1-10 bar and give output of 1 bar, inside the black screw cap
is just a spring.

https://www.claber.com/uk/cod/91040/Block-system-Rainjet/Pressure-reducer

But when connecting the PRV, I can't get any get flow out of it, with
tap wide open (impossible to stop the flow with my thumb) very likely
the input pressure is exceeding the 10 bar maximum, so the PRV is
shutting off? But I when I close the tap down to a trickle (easily
stopped with a finger) I still got no flow, below the 1bar minimum? No
amount of adjusting the tap would get flow through the PRV.

AFAIK 1 bar is 10m of head, any reasonable way to estimate the supply
pressure? With the tap fully open, at the 3/4" connector the full flow
spurts about 18" vertically, restricting the flow as much as is possible
by thumb the jet will reach about 4m vertical ... what ball-park
pressure have I got? I'm looking to get another PRV to fit ahead of the
Claber one but don't want it to be too much of a guessing game what spec
I need ...