under cunstruction flat purchased,

Dear Sir, i have purchased under construction flat in Aug 2013, at the time of purchase builder has promised that work is almost done and possession will be in dec 2014, however the work was not compelted and possession has delayed and builder has again promised for march 2014 onwards. mean while i have not paid few installments because the possession was delayed .builder has called in sep 2014 mentioning that the flat is ready for possession and clear all your dues or else we will charge 18% interest as a penatly. i visited the place in nov and surprise to see that there was not water connection and no electricity, now in march 2015 also there is not electricity in the flat and builder is saying you have to pay interest as panelty, i have registered my flat just last week and paid all the dues, but still the builder says he will not give possession unless and untill the interest is paid on the delayed payment, kindly let me know what i can do in this situation,can i approach consumer forum for the same