Many thanks for the advice. At the moment both sets of preferences have the same values:
{author_sort}/{title} - {authors}
... so at least this is consistent.
I'm wondering if the slightly odd folder structure arises after "russification"...
At the moment on my PRS-650 my books are stored in one location only:
S:\database\media\books
... so at least I'm in the default place.
There is also, and confusingly, a (now empty folder) at S:\media\books.
As it's empty and not causing problems I'll work on the "if it ain't broken, don't try to fix it" principle - and accept a little uncertainty in my life
Best
C: