Partial occupancy certificate

Dear All, I had purchased one flat in Kolkata. (specifically in Newtown, NKDA). Builder handed over the flat to me and when I specifically asked for the Occupancy Certificate, after some delay they did provided me a copy of the same. I found that it was a Partial Occupancy Certificate from the authority only meant for the block where my flat belongs to. Now I also noticed that it was written in that Certificate that the validity of the certificate would remain valid till 15-Sep-2015. It was also mentioned in that letter that the builder should get the permanent one before the expiry of the partial one. Meanwhile I have registered the Deed in my favour and also took over the possession. Now when I wrote to the builder and as usual they are not responding to me. I need help to understand whether in Kolkata Occupancy Certificate is mandatory to have? Is it now illegal if I continue to occupy the said flat as the validity of the Partial Occupancy Certificate has aready expired? Can I ask the builder to submit me the re-validated copy and take legal action against them for non-compliance and unfair trade practices? Please suggest what best I can do here. I have sent multiple emails to the builder, but they are simply not responding. Can I take legal action against them for not providing me valid OC / Partial OC? Thanking you.