With all due respect to the clarifications above, in my personal view, if the payment (along with interest) made in the month of April 2016 is pertaining to March 2016, it should be shown in the Oct 15 to Mar 16 return. Reason being:
1. Payment made in April 2016 is pertaining to Mar 2016 itself
2. For instance, ST for the month of Feb 2016 is payable by 6th March 2016, even though it is paid in March 2016, in the ST 3 return the payment is shown for the month of Feb 2016 in the paid sheet and challan sheet. Similarly, Payment made in April 2016 pertaining to Mar 2016 should be shown as for the month of Mar 2016 in paid sheet and challan sheet and interest to be disclosed separately.