So I started playing around with the controller. Initially I installed it to measure manifold pressure only from the spare nipple (without the solenoid and without capping tvsv) so I could see how it was running. I was seeing readings of 4-6 psi, with a maximum of 7.5psi wot in third.
After I installed the solenoid and plugged tvsv, it was comfortably reaching 10-11psi. I still need to tune it properly and don't want to push it without a FCD but it made a significant difference.
I suspect the cause of the low boost may be that a previous owner had a boost controller installed (possibly Apexi AVCR given disconnected splicing into ecu) and disabled tvsv, and then removed the boost controller prior to sale so that it was running on waste-gate pressure only. I'll have to have a look underneath to see if the tvsv coupling (blue apparently?) has been disconnected.
Does this sound like a plausible explanation? Is there anything else that I should look into that may have caused the previous low boost? There's no engine codes so it shouldn't be a knock / limp mode issue. I'm 92% sure that my guess is correct, but don't want to cause damage if there's another issue.
Side note - very happy with the Blitz SBC. The graph display and record functions are awesome and it's super easy to setup and use. I was planning to get the OLED GReddy Prospec B but can't turn down a gumtree bargain.