I tried what you did, switching from V.Master to A.Master and back again - and that changed the options of the ‘sources visible on ml’ to add line in.
so now all works as it should! I would never have thought of trying that!
For completeness, I have a ML Powerbox connected to the NL/ML converter and a Beomaster 5 as well as a BeoLink passive connected to some Beovox 1
thanks for your help.