the thick and thin of it is you cant prove it, ceop and the police cannot give out details (even under sarahs law) if the offender can prove that he is not in contact with a child, the police cannot arrest the individual for being on facebook. whilst facebook do not allow R.S.O's, on their site, it is not a criminal offence in the uk to register, as facebook is an american company and american law has no juridstiction in the uk, besides, however if you can prove the person is in contact with a child then the police may decide to give you the details, but you would not be allowed to share this information with anyone as you would be required to sign the offical secrets act, to maintain the safety of everyone involved, also they would not tell you his address, his name if he changed it or indeed the actual offence commited. i know this may not be a lot of help to you but i hope it has clarified the situation for you.