P45 and self assessment

I'm need to file my self assessment form but have queries relating to my main job. My main employer gave me a P45 when I left but the leaving date - 31st March 24 - given on it falls in a different tax year (2023/2024) to the final payments (which were provided after I left in the new tax year 2024/25). Two contacts at HMRC (one a specialist apparently) plus my accountant has said this is wrong because it will show payment for a year when I wasn't employed by them and that I should also have had a P60 for the tax year I actually left. However the employer has said they spoke to their HMRC contact who said no P60 was required for the year I left and the P45 for the subsequent year is correct. Any thoughts on what to do and/can anyone share the law/rules around this point? Will it affect my self assessment?

Can anyone help with this please?

Hi,
Normally you would receive a P45 when the final payments are made. If this was in the 2024 to 2025 tax year then the P45 would show details for 2024 to 2025 and you would receive a P60 for the 2023 to 2024 tax year.
If you think the pay and tax details supplied by your employer are incorrect you will need to contact the payroll department to review or amend. 
Thank you.

Thanks, are you able to share the particular regulations or rules that suggest this? The employer is insisting their HMRC specialist has advised that what they've done is correct and are shutting the conversation down. It would be helpful to know how this will affect my self assessment?

https://www.gov.uk/paye-forms-p45-p60-p11d/p60 says "If you’re working for an employer on 5 April they must give you a P60." Perhaps the difference of opinion between you and your employer arises because they consider your employment finished on 31st March. What kind of payment was given after 5th April? e.g. was it regular salary / pay in lieu of notice / reimbursement of expenses / compensation? Was the amount of the late payment included within the totals on the P45, or not?