backpay to correct previous underpayment how to resolve

Hi My employer underpaid me for several years after I took on additional work in Jan 2020. They have now agreed to pay the money which I am told will be arriving in this month's pay but do not seem willing or able to allocate this to the correct tax years via PAYE system. I do complete a self assessment as I have some rental inclme as well. How do I approach fixing this across all these five tax years? Please be as detailed and specific as possible with which questions I need to complete in the online self assessment system. I really appreciate your help and advise.

Hi,
If your employer is repaying you these underpaid wages through your PAYE account, we may not be able to amend this for you until the end of the 25/26 tax year.
When this tax year ends, write to us confirming how much of this income is to be attributed to each previous year, and we can review your records.
Thank you.

Many thanks, what is the best address to write to? Thanks

Hi grandmasterglitch
Please use the options at : Income Tax: general enquiries
Thank you.