Backpay From A Pay Award Covering Several Years

I work for a local authority and after a lengthy negotiation colleagues and myself were regraded to a higher level which saw our grade increase. The negotiation took several years and after the outcome the employer looked back 4 years - calculated hours worked, overtime etc throughout the years and came to a final figure which was paid to all during the 2023/24 tax year. The lump sum we were paid last tax year took me above into the higher tax bracket and as a result I paid 42% (Scotland) on a portion of my annual salary. My feeling is, as the earnings from the back pay were from the last 4 years then the taxable income should be for the relevant year. Had this been done then I would not have come anywhere near the 42% bracket and would have seen myself taxed at 21% for each period. I spoke to HMRC who asked for proof from my employer that the backpay was over the multiple periods and the portion attributed to each tax period. I submitted all of this yet received a letter today saying simply my tax was correct. For clarity had the earnings all been through 23/24 then the amount paid is correct - The point being it is over several years that seems to have been ignored despite an extensive dossier sent to HMRC There was zero explanation to say why my theory was incorrect. Taxing it all over one period feels unfair, Can anyone clarify if this back pay could / should be apportioned to the correct periods - If so what is the best way of moving this forward?

Anyone got any input on this. Need to know if I am fighting a losing battle here or if I should dig in and fight for a positive outcome.

Similar situation related to teachers in Scotland. I was particularly interested in the opening statement - Can anyone confirm this is a true statement? "Under UK tax law, income from employment is taxable at the date on which an employee becomes legally entitled to the payment."

[External link removed - Admin]

Hi Derek_F_1968,
You will need to write to HMRC at H.M. Revenue and Customs Pay As You Earn BX9 1AS and make a claim for 'arrears of pay'.  In your letter, you will need to provide a breakdown of the payment you received and how it relates to each of the earlier tax years.
Please have a look at the guidance at PAYE70023 - PAYE operation: aspects of PAYE operation - employer: payment of arrears of pay for closed tax years and EIM02530 - Employment Income: arrears of pay and awards under the Equal Pay Act 1970 for more information.
Thank you.