[Bill UK]

It doesn’t sound like the turbo has gone down, as this started straight after you took it in for the starter motor replacement, I would more than likely suspect someone has connected something incorrectly or forgot to connect a hose up. Could be a split hose or a hose connected in the wrong place, or a hose not connected. The pic below shows where the stock gauge connects to the pressure sensor via an electrical connection. Aftermarket mechanical boost gauge could “ T ” in anywhere along the hose (yellow arrow) that connects to the sensor. Unrelated to your problem, but interesting all the same Link