P60 and final 2023/4 payslip discrepancy

Can anyone confirm which figure will be used to calculate any NI credits to my account? My P60 figure is lower than the NIable pay on my final 2023/4 payslip - this seems to correspond with an amount of back pay I received when I started working (prior to that I hadn't any NI credits for that year). This may influence how many "missing weeks" of credit I have for that year. Should backpay normally be included on P60?

Hi,
National Insurance contributions are deducted on all earnings above the primary earnings threshold.
Please see further information regarding National Insurance rates and categories here - National Insurance rates and categories
You can use the National Insurance calculator to check if your National Insurance contributions have been correctly calculated - Work out an employee's National Insurance contributions
Your total gross pay should be recorded on your P60.
If you think that your employer has deducted National Insurance contributions incorrectly, you should contact your employer who will be able to advise you.
Thank you.