RESOLVED!
A further poke around the engine bay (stone cold this time), reaching down beneath the exhaust manifold to the rear of the turbo I found the actuator rod flapping around. The retaining circlip had gone awol and the rod has disengaged from the wastegate arm, meaning the turbo has not been boosting as I suspected.
Reattaching the rod has returned performance back to normal, with the lovely torque fill returning, and MAP readings (courtesy of a borrowed ScanGauge reader) showing at around 22PSi at full throttle.
