If you have a booking form along with receipt for having paid the amount that is proof that you have entered into an agreement of purchase from the builder.
Date of possession was 2012, builder has breached the date, he is liable to pay delay compensation even in the absence of a written agreement between the two.
Issue a legal notice, give the builder 7 days to pay the delay compensation, failing to do so, file a complaint before the consumer forum. court will award delay compensation to you.
At the time of filing of the complaint, implead the bank as a necessary party to the proceedings, that would be sufficient.