[Koha] German Z39.50

I'm looking for a German Z39.50 server. The Deutsche Nationalbibliothek has dropped Z39.50 in favour of SRU. I can connect to Gemeinsamer Bibliotheksverbund (GBV) server z3950.gbv.de on port 20012 (UTF8), requesting Marc21/USMarc, using database gvk. This will bring me in (at least some) results for ISBN (e.g. 341601331X) but author fails. Title might work e.g, it will find title 'Der Flieger und sein Flugzeug' (no quotes), but not the author 'Sohn' or 'Sohn, E' copy/pasted from the Marc record, and not find title AND author (which is what our cataloguers normally use.) Has anyone got the full details of a working German Z39.50 for Marc21 biblio imports? Many thanks -- Paul