File complaint against builder before RERA and seek orders to direct builder to deliver possession of flat to you and waive interest charges as he delayed possession of flat and bank could not disburse funds due to builder fault 

For this dispute along with the bank and the Builder you need to approach state consumer dispute redressal Commission making the bank as well as the Builder party to the case it is primer face I clear in your case that the Builder has to give the interest amount to the bank during the period your subvention scheme was in application as well as after the booking in 2012 the Builder has to give the position of the property in 2015 Howrah the project is delayed by another four years the Builder is liable to pay the penalty as per your builder buyer agreement along with the interest to the bank because you are making the bank as a party so bank will come with its written statement in this regard along with the Builder to the court to decide this matter

A complaint must be filed against this builder in the rera authority.

Regards 

See you can file a complaint against the builder to waive the holding charge as builder failed to pay interest it was deficiency on agreed terms on his part. 

You can object for the same and deny it. You can complaint in consumer court

approach before District Consumer court. 

File case in rera but before that try permanent lok adalat if you have it nearby. 

Since this is under subvention scheme, you cannot be held responsible for the non-payment of the loan amount by the bank.

The builder should follow it up with the bank instead of asking you to pay the exorbitant charges.

You can issue a legal notice demanding handing over possession o property.

After that you can plan to initiate legal action as per law.