Thanks Alex, in fact you were onto a solution which I have now found, maybe points to a minor bug with Media Server.
By default Media Center seems to create its library in User/documents & settings/application data/j river. However soon after installing I decided I wanted a single common database on a separate drive which I set up using Library Manager, no problem. I also deleted the reference to the now redundant original database - so there should in thoery only have been one database.
However Media Server (which remember can run on its own without starting Media Center) was still referencing the original now orphaned databse.
I've deleted these orphan databases under all user names and so far all is well.
Interestingly I spotted the answer when I noticed that on the server PC media server was reporting no accesses by any clients even though the client was running.
To solve the problem Media Server should be configurable to address a specifed database, or it should automatically address the default databse on the server.