Mutation process

We had an inherited property of around 7 Katthas put to promotion against a POA for development with a promoter way back in 2003 on 61% to 39% Basis. The land was initially against my grandmothers name but post her death in 1992 the legal heirs were her 4 Sons and 3 daughters. With the death of my father in 1998 ( her eldest son ) we namely- myself, my Brother and my mother also became party to the legal heir ship and our names are featuring in the Municipality Assessment roll against the property in question. We are having a 957 Sq. Ft Flat now post development of the property but the promoter has not issued a proper possession letter in our name, to make the matter worse the copy of the POA for development that we had also stands missing ( the POA is highlighting our names as legal heirs ). Under such a scenario how can we go for a Mutation of the said 957 Sq. Ft Flat, the POA is not there with us and even the Possession letter from the promoter is also not appropriate. Please advise..