Pete(V8), I managed to undo my plunger extender with a 7mm socket and a pair of long nose pliers to hold the lock nut in place. it only extends about 2cm so I think should be kept for fine adjustment.
For anyone else using an E30 master cylinder (which I don't think has been tested yet), you'll hit a problem with the length of the push rod bit being too much
Basically the seat inside the Servo is 25mm deep with the adapter plate installed. From the E30 fixing face to the end of the M/C push rod uncompressed is 50mm so a few options could be
1. Cut 25mm off the end of the push rod. Easiest way to proceed but it depends if there are seals 25mm inside the M/S that would leak if you press the brake pedal down too far.
2. Create a further adapter 25mm thick to stand the M/C off the servo or layer up a total of 4 of the existing adapter to make the trim cut on the M/C just 10mm. This may push the M/C too far forward so it fouls the side of the bonnet or side panels
3. Stand off spacers for the M/C. As far as I understand it the servo is a sealed unit so the M/C to servo interface doesn't require an air tight seal
4. chuck the BMW M/C and buy a ford one
5. something else entirely (open to suggestions)
I think trimming the M/C pushrod is the most likely solution but looking at Mikes photos, I'm still surprised this is an issue as the ford one looks very similar
[EDIT] no it doesn't, it quite clearly doesn't have the protuding push rod