Possession of flat

Hi, we (joint property of my uncle & my father) had a house whose land was amalgamated with another land to construct a flat. According to deed we got a flat on 1st floor and half flat on the fourth floor. We bought rest half part of that flat. It is said on the deed that we will take the possession of the flat on mutual understanding. We yet to pay our last installment and have not got flat posssion or flat hand over letter from the promoter. Though they have handed over keys to both of us. My uncle has already occupied the flat on the first floor and started living there. We have started interior of the flat on the fourth floor. Our relation is not very smooth. My uncle is delaying the payment of their part of the last instalment. So we could not pay the last installment yet. They are also reluctant for allotment of the flats. He is saying as we are landowners so we do not need any further registration. He will think about allotment after getting possession letter. But considering his some previous activities we have doubt that he may create problem during allotment. In the mean time we are spending money for interior of our flat . So our questions are how can we secure possession of our flat for which mutual allotment has not done yet? Secondly if we have to do registration of our property or as we are land owner so we do not have to do that? Pls help.