Consumer Redressal

I have purchased a flat from Nirman Realtors and Developers Ltd. Recently I have received letter from them demanding interest for delayed payment. I checked all records and found that I have made all payments well in time. Except one payment which was paid within 15 days from date of demand letter. But I did not receive any letter for receiving this interest due letter. I have written letter to the builder seeking information for delay payment but they failed to give the details. Also they assured verbally that it may be wrongly sent letter but did not give any written reply. Also again the sent letter for interest. The again I sent letter seeking information and removal of wrongly charged interest. But they have not replied to any of my letter\'s for which I have acknowledgement. But again they have letter threatening to cancel the agreement. Also the builder has taken Rs.127480/- as excess amount from me. Kindly let us know what action to be taken now ?