Service tax payable at invoice date or current date rate?

A few of my clients have taken very long to settle bills (4 years and 1 year). In the first case the Invoiced service tax rate was 10.3% and in the second case it was 12.36%. The current rate is 14.5% and does not include the Education Cesses, but only Swacch Bharat Cess. As aproprietorship firm I pay tax on collection basis. Should I pay at the invoiced rate or current date rate and recover the extra from the client? If interest on late payment is to be paid in this case, how is to be calculated?

Thanks in advance.