allocating pay arrears to previous tax years

Our pay award that was due in July 2022 was decided in May 2023 and all the arrears - backdated to July 2022 - were paid with October 2023 salary. This was a sizeable lump sum. From the information I have seen on HMRC website and in several different answers in this forum, we can ask HMRC to allocate a portion of the arrears to the 22/23 tax year, because "Legally, an employee’s tax liability on a payment of arrears arises in the tax year that the employee was originally entitled to be paid the extra amounts, not in the year that payments are eventually made". This would be beneficial for me and other colleagues because of tax bands and thresholds - I have to complete a tax return for the first time because my taxable pay for 23/24 was above the child benefit threshold and I will have to pay most of it back. We need the correct breakdown of figures from our employer to request this correction from HMRC, but HR are refusing to provide them. They say that because the pay award was confirmed in 23/24 tax year we weren't legally entitled to the higher pay until that date so the statement in the quote above doesn't apply, even though the higher pay was backdated to July 2022. I've read everything I can (and can understand!) on this subject and I haven't found anything about the date when a pay award was decided being of significance. Are the HR department correct? Is there a document that covers this one way or the other that we can reference? Thanks

Hi,
Please see guidance here:
EIM02530 - Employment Income: arrears of pay and awards under the Equal Pay Act 1970.
Thank you.