Thanks -

Is this information available anywhere ? Or is it just industry that has it .... ?

NB it's a little bit different to what Avis at the airport told me
Also, I considered Budget first, and called them (well, Budget US) - I went through the booking process and was quoted EUR 30 cross-border fee.

Edit: found this (eventually) on the Avis.sk website - in Slovak, I presume (google translate)
"Entry is not permitted to countries: Albania, Bulgaria, the countries of former Yugoslavia (except Croatia), Russia, Ukraine, Romania.
Entry is allowed only for Gr. Vehicle P, A, B, C 1, C 2, C 3, C 4 to countries Hungary, Poland."

So now I have 4 sets of differing information to work with :-)