Delayed interest charged by a buidlder

Dear Sir, Please help me with the following problem. I booke a flat in Noida Extension in Noida/UP. The project is in time.The builder has sent me final demand in which he has charged me Rs. 50,000 interest @ 18% on delayed payment, which he has calculted from the date of demand letter till the date of payment received. It has pinched me as I don't have any provision for this in my tight budget. On investigation, I found that few payments were delayed by by bank who has given me loan. In some cases delay was there due to wrong demand letters sent and wrong service tax charged, whoch was corrected and then paid but builder has charged the interest for full amount. Further the builder has not taken into account the transit period. I have made a written request to the builder but they have turn down my request. Please advise if I have a strong case to go to consumer court.I want to fight on the following points: 1. Delay which were happned due to bak disbursement. 2. delay due to wrong demands sent 3. Trasit time and grace period of 15 days has also included in the no. of days for which interest has been claimed. 4. The demand is time barred as it is rasied after 3 years of payment. 5. The builder should have added the interest arrear in the next demand letter which could have prevented this situation and the very intent behind this is to extort amount from the buyers. Please advise what should I do. Thanks & Regards P.C. Joshi