Refund after Cancellation of Alloted flat by Promoter

I have booked small 2 BHK flat in Akshaya Tango project(Chennai) in Feb 17. I was shown the size of each rooms/bathroom/kitchen in the flat in their booklet. Based on that i gave them 20% of amount for allotment. Allotment was done in Mar'17 and got the letter from builder that so and so flat has been alloted to you. Post that I got the entire agreement letter at home and in that i found that the room size and bathroom is smaller then what they shown me at the time of booking. After that i sent out an email to Builder for cancellation of flat giving them the reason. Since Match 17 I am following up with them over email and phone for refund and they are just giving excuses(demonitization, PNB fraud) . I have email from them to pay in December and then in March. Now they have stop responding to me over phone or email. I am planning to fill Grievance form with TNRERA now. Is that approach right or what should i do now. I also need the interest on the money which they are holding for year now without any reason.