Refund from builder

Hi there I have booked a flat in march 2015. I have paid around 15% down payment as booking amount ie Rs 4.5Lacs, the proposed flat was under construction. I have signed Builder's Buyer Agreement also. Builder didn't provide me papers till the time i paid 15% amt otherwise i would have approached for legal opinion but when i approach for NEC of that property i get to know that the property has already been collateralized with some other bank and the builder failed to provide me NOC from that bank. I am working in nationalised bank and already told the marketing person that i will only prefer my bank for housing loan to get employee benefit from my bank on housing loan but due to inadequate documents my bank refuse to accept the loan proposal as the property was already collateralized. And since march there is no progress of construction and when i applied for refund with interest they are making excuses and making false promises that we will pay the amount but still i didn't get any refund. How can i get my refund and what should i do now? Shall i file a case with consumer court? What is the procedure and how much time it will take to get the refund along with interest? Please guide me.