Interest charged by L&T Realty for delay in payments

I have purchased a flat in a L&T project in Mumbai. The builder has extracted 95% of the cost 15 months before the planned date of delivery. The delivery was scheduled for April 17 which now ill happen in Aug 17. Builder has a 6 months delay clause which covers his delay. At time of delivery, L&T is not willing to give possession unless I pay interest for late payments done in the past. My question therefore is as under: 1) L&T took 95% of the money in year 2 and made me wait for another 18 months before the actual possession. Is this fair and legal? 2) L&T has a 6 months delay clause in its favour where as a customer cannot delay payment even by one day. Is this fair and in line with natural justice? 3) The cost of the flat includes all the amenities of the complex which will only be ready for use in 2019. In other words, I am expected to pay 100% for a product that is only half ready. In such a scenario, is is fair and legally correct to charge interest? 4) None of the demand notes/invoices ever claimed an interest payment. In such a case, can L&T claim interest because its part of the agreement?