Debtor denies payment after delivery of goods & services

We are a small firm operating out of Durgapur. We have supplied & installed certain goods to a client. Now after about six months of completion of work the person through whom we got the order (and he was also the co-ordinator between us & the owner) refuses to pay the balance amount due (he had paid about 30% of the total billed amount through bearer cheque from his own A/c & cash on behalf of owner). My questions are as follows: 1) What are the legal steps to recover the debt. 2) We have a email trail linking the owner & the person, mentioning our supplied goods & services , and with attached invoice, can this be used as evidence? 3) We have recorded conversation of acceptance of our work from the middle man as well as the owner, can this be used as evidence?