Builder and Buyer responsibilities

Dear Sir,
I am from the city of Lucknow where I did buy a flat from a builder in Feb 2005. It was supposed to be ready for possession in Nov 2005 itself but the time is not mentioned in the sale deed. Since I am NRI I applied for a loan and the flat for financed by a nationalized bank. Bank had paid almost 86% of the flat cost to the builder within one month from the date of sale deed signed. Flat registration fee was also paid to the builder and for the rest of the amount the cheque was given in builder's name with a verbal understanding that this amount will be paid at the time of possession. The builder could not deliver his promise and the flat was not finished for possession until Nov. 2010, though it is yet not ready to the full. In between I had been having a regular meeting with the builder on my every visit and was discussing the delay and the loss of rent which we were incurring just because of his fault. In the sale deed it is written that if the buyer does pay 03 consecutive installments he would liable to pay an interest @24% (which I believe does not apply to us as ours was a bank financed case) but in the other para this is also mentioned that if the builder fails to deliver the flat in time he will pay a flat interest of 8%.
Now when the flat is almost ready for possession I asked him to provide us with the final outstanding so that I can clear it and take the possession. The builder has responded with a statement which indicates an interest of 18% p.a. on the amount which was not cleared and which was given to him in the shape of a cheque. According to him he has charged only 18% instead of 24% just to compensate the losses in terms of rent for all these years. This act of him is making me to pay almost double of what I had thought, because I did not have the slightest idea of his charging interest on the amount pending in the shape of the cheque and too happened because he did not deliver the flat on time. And on top of that he is adamant on his charging interest.
Whereas I think he should have paid us an interest of 8% p.a. on the paid up amount i.e. the amount paid by the bank + the registration/booking amount.
I need to ask your goodness as what would be our legal status and should ask the help of consumer forum. And if yes what are the requirements and how much time would it take.
Will appreciate your prompt reply as it has become a nightmare for me and that too when I am fast approaching to arrive in India.
Regards,