I see - I guess I just needed a little more effort on the rubber mallet.
The manual says "remove from rear axle shaft flange. Loosen tightly seated brake discs from rear axle shaft flange by tapping
lightly with a plastic hammer"
I guess lightly is a relative term here.
Anyway - I'd found in the manual that you need a special part:
126 589 02 09 00
On
http://www.startekinfo.de/etools/content/tn.jsp?prefix=100 it says this is a 12 pointed 30mm socket.
Thanks for the information about the disc removal.