Hi George,
I have got a BMW to do soon which is the same or similar Garratt turbo. I have a copy of the BMW fitting instructions which tell me that it has not got a position sensor and feedback is via the boost pressure sensor. This means that no base setting is required. I have been told that if the turbo and actuator are separated the whole assembly must be replaced. I am not sure how correct that is but it may be the case that the company Mike is suggesting repair them as a complete unit. The one I have to do is because of a noisy turbo not an actuator fault. What I am a bit confused about is that they are controlled by PWM, but vane movement is done via a worm gear? Seems an odd set up to me.
HTH's
Regards, Bryan.