Good news, I disconnected the actuator arm today and the wastegate clonked shut - the arm was holding it open, hence no boost! I took the end adjuster off and it was a little bit bent, so I straightened it and refitted it, adjusting it so the wastegate is firmly shut now. Will know tomorrow if that has fixed it or not. Just glad the wastegate wasn't jammed open or anything, it moves freely by hand so either it just needed adjusting or the actuator itself is goosed. If it is I can get a new one from Forge.
Electrical upgrades will be done next week and hopefully she'll be back to all ok again.