PSSTTT.... the screw is a T20, not a T15. I've seen it referred to as a T15 in several articles about doing the elastics so I tried a T15. The T15 catches just enough of the splines that it can be used to unscrew the screw and screw it back in, but it's WAY easier to do with the correct size Torx - which seats itself quite securely in the screw Torx socket, making it much less likely the screw is going missing while working with it. I'm guessing some time long ago in a land far away someone posted it was a T15.. and the mistake was started and just continued.For some reason only hollow centered bit worked for me (t15):
I'm glad i've bought in the past a set of various bits shapes - this is a must with the German car.