Builder is asking more money and not responding

I need your suggestion regarding the trouble that my builder has started to give. I have a bought a flat inb ARihant Garden , Rajarhat and below are teh issues 1) the posession was delayed by more than an yr. there was a clause in the agreement where the builder had to pay intrest to us if the flat is not completed in time. 2) Gym - builder had promised to make a gym but finally he made teh gym in teh garden area which was not part of the plan or brochure. he did not even inform us on this change in plan 3) Rs 30 per Sq ft + ST was already taken from me for Electricity charges for the flat during possession. now he has sent a new demand saying Rs 12000 more has to be paid for . 4) Maintainance charges @1.5 per sq ft were already paid in advance for 1 yr. now he is again asking for maintenance charges and says that the money i paid was advance deposit and weill be returned back. 5) He never replies to any mails that we have sent . like me there are other flat owners in teh same complex. what can we do. please suggest