Hi,
You would ignore the payments made by your employer, as you cannot claim tax relief for those payments. You would declare your gross payment, your payment and the tax claimed back by Nest, in box 1 of page TR4 on SA100. This will allow you to claim any additional relief you are due.
You can amend your two previous tax returns, 2021/22 and 2022/23, and claim overpayment relief for the earlier two years, by following the guidance here:
SACM12150 - Overpayment relief: Form of claims
Any overpayment relief claims for 2019/20 received after 5 April 2024, are out of date and cannot be claimed.
Thank you.