Pending of balance payment of final bill

My company has executed civil contract work under EPC Contractor of mumbai during 2005-2005, the place of contract work ( cause of action) was in district Faridabad. My company had completed the contract work & received the interim payment ( on basis of progressive work completed monthly & raise of Running Account bill ) through bank cheque on regular basis. In final Bill the EPC Contractor hold our Rs 8,20,000/ & informed us that he will pay within month times . But as on date they have not paid us. Every time they have given assurance that they will pay our balance payment. In year March 2012 ,they have sent an email to us, for the confirmation of our outstanding Balance payment along with our company ledger account in their books of account for confirmation of outstanding balance payment . Then we have contacted them in year 2013& 2014. Every time they said yes we will pay you payment. But now they are not picking us our phone as well as no any reply. Pls let me advice . We planning to sue the money recovery suit against them.