Another 530 miles competed. This time at 51.4MPG.
I've played with the bleed tap a bit and got the boost gauge to top out at 7PSI instead of 10PSI, but that was when the gauge was in-line next to the bleed tap. I've since re-positioned it to sense just next to the take-off from the manifold, so it is sensing the boost in the manifold, not at the bleed tap.
Since I positioned it there, and despite opening up the tap a fair bit the boost still hits 10PSI and sticks there.
Not sure why this is, the ECU should be allowing the boost to rise past 10PSI, as it shouldn't see the full boost because of the tap.
A re-think is needed.