Recovery of service tax arrears from customer

Dear All,

I have a Query regarding the above Topic. Our Co. Started Business of IT Services in Oct 2011 and started getting the Payments for such services form Nov Onwards. Up to 10.00 Lacs of Fees we did not recovered the Service Tax. But afterwards from Feb Onwards we charged service tax and raised invoices charging service tax. We also recovered the Service tax not charged from Oct to Jan 2011. Now my question is:

1. For the arrear recovered from customer, how will in enter the amount in books and do we need to raise another invoice claiming only service tax for previous periods or we need to modify the previous invoices. But we raise the previous invoices it will be default of non payment of service tax on time and will arise a penalty and interest.

2. How will i show the arrear recovered from customer in Service Tax Return.

Hoping for an early reply.