I have similar problems, someone has claimed copyright for an arangement of Bach that is, to the best of my limited knowledge, in the public domain as far as the music is concerned, and my own as far as my performance is concered.  

It is not clear what the basis of their claim is or where it is meant to apply.  Indeed, I am not even sure from the info suplied if it is the music or the performance they are laying claim to, so to be given a stark choice between acepting and totally refuting their claim seems rather crude.  I think it is a false claim, but I could not be sure as I am not an expert on copyright law and do not earn an income from my performance of classical music that allows me to aford one - esp as I do not stand to recieve any royalties what ever the case.