[thomastee]

Bob, thanks very much for the reply and the picture of the valve, very helpful indeed!
Here's my update:
I hooked up the gauge just to do the valve closed system pressure test and the pressure was very low (2 bar). I then started playing around with the pressure valve, took it out to see if anything was binding - it wasn't. After playing around, trying to understand what adding shims might do, it turns out that the valve worked better (pressure higher) with fewer shims, i.e. more loading on the valve spring. So I took all the shims off but the pressure still wasn't enough, so I started adding a few tiny spacers between the spring and the bolt so that it would get more loaded. Well, that started sending the pressure to the right region and I got just over 4 bar with as much loading as I could cram onto the spring.
Thinking about this a bit more, it strikes me that my pressure valve doesn't look like your parts diagram Bob, in that the sharp piece the arrow is pointing to is missing. I certainly didn't see anything like that. Actually, the parts diagram for my car doesn't have that pointy piece but it does show a smaller shim in between the stopper and the spring - which is essentially what I'd added with those few tiny spacers.
I think I'm onto something here! Now if only I hadn't lost the line fitting for the banjo bolt I could start the car and give it a whirl...