I've T'ed in the gauge to see what it's getting at the moment. This has been t'ed in just before the ECU, so will need to be re-t'ed further away from the ECU, if I put a bleed valve on it, to up the boost the engine is getting.
After all the mithering about boost gauge mounting on here recently I put a lot of thought into the fitting up of the gauge:

I may need to remount it at some point, as it's going to annoy me because it's slightly on the wonk. I'll put the effort in on the mounting next time to get it right.
Or it'll be fine.
Car makes a solid 10PSI of boost, which indicates that the N75 valve is working fine and the set up is reasonably healthy, but it needs turning up, clearly.