Society not transferring share certificate

Hi all, My dad bought a flat in a single building society on collectors land, in 2006. We paid the collectors fee and the previous owner and moved in the house without paying stamp duty. The society kept telling us that the stamp duty hasn’t been paid so they cannot transfer the shares and put our name on the buildings members name board. Finally this year we managed to pay the stamp duty via amnesty scheme. I gave the document to my society office with the share certificate and asked them to do the needful. The society raised an objection on the fact that the sales deed is not registered i.e the last page does not have my mother (current owner) and the previous owners picture and finger print. Now we don’t know where the previous owner is and if he is alive at all. Secondly I spoke to a few agents who told me that the registration process is shut due to some court case since 2007. I relayed this information to the society committee but they refuse to acknowledge the issue. This AGM they gave me a letter of a property lawyer stating that the registration must be completed for the transfer of shares and to become a Member of the society. They also scared me saying that the previous owners family member can lay a claim on the flat since registration is not done. Now I have all the other necessary documents. Except for the registration of the sales deed which to my knowledge is not operational. What can I do?