Can you tell me why you received salary for 2008 to 2013 in 2013 please.
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I will finish drafting my answer and then call you if that's OK. I'm almost done.
If you look here, you will see that payments on pension arrears will be taxed in the tax year of actual payment as your arrears of salary have but that the taxpayer can have the pension arrears assessed in the tax years in which they were paid with the tax deduction in the actual year of payment being used to cover those liabilities.
Now look here and here. Payments on salary arrears are taxed in the actual year of payment. It says that legally, the tax liability arises for the tax year in which the salary should have been paid. If you look at page 22 here, it tells employers to allocate arrears of pay to the tax years in which they should have been paid but as you are retired, they cannot do that. Therefore, you need to write to HMRC to ask them to allocate the salary arrears to the correct tax year (as with arrears of pension payments). You will need to get the payment you have had split between tax years by your forner employer to send to the tax office.
I hope this helps but let me know if you have any further questions.
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