Overpayment for 16/17 and 17/18 tax year

I have had an overpayment going over few years to include 16/17 and 17/18 tax year. I have corrected using software were possible but for 16/17 and 17/18 I am not able to.
Will letter directly to HMRC be acceptable or we no longer can amend figures for employee and employer for these tax years?
Is there any article on HMRC that explains the reason for that please?
Thank you

Hi MBanjo
Yes, if HMRC currently hold incorrectly liability totals for the 16/17 and 17/18 tax years, you can write to HMRC confirming the correct breakdown of Year To Date (YTD) totals:
•    tax, 
•    employers NIC, 
•    employees NIC, 
•    student loan etc. 
Please note that there are time restrictions on claiming overpayments – 
PAYE47001 - Employer returns: employer return - overpayments: introduction - HMRC internal manual
Thank you