Ok so I was all set to rip it out. After having done my first set of control arms and ball joints I figured the job couldn't be too bad as a DIY.
After waiting for Bilsteins to come into stock now for 2 months, a comment from AussieXer re swapping out the bag only became an option. A new bag was only $225 so was worth a punt. A couple of tips and links from AussieXer and it was on.
Wrestled with the removal for over 2 hours as the old bag was seized on the shaft. As I said elsewhere, the gentle persuasion of the manual impact device, rotary penetration device and surgical cutting device meant I was gonna win in the end....
Getting the new bag on was then a bit tricky but eventually with lots of silicon lube and more usage of the manual impact device it was whole again.
So after reinstalling the strut and resetting the EHCII module... the moment of truth... success or failure...
and success... after reset it just simply rose up and all was good with my BMW's again...
Wife is well pleased to have her car back..