Redundancy - amount free of tax

I've just received a letter from HMRC stating that I owe them money for last year, but in fact, they owe me due to an error in calculating my tax-free redundancy payment and its impact on the follow up tax codes. I attempted to address this through the self-assessment process, but there doesn’t seem to be an option for this particular scenario—redundancy. Does anyone have any advice on the steps I should take? How is it possible that the fact of my role being made redundant is not visible to HMRC? I’ve tried calling, but getting through is proving impossible.

Hi,
If completing a self assessment return you need to show the information on the SA101 additional information page.
SA Notes regarding completion of the SA 101 can be accessed at: Self Assessment: additional information (SA101)         
Redundancy is covered at Page AiN 3 and Box 5 advising:
 This includes: 
• redundancy pay 
• post-employment notice pay 
• compensation for loss of office Include the amount of taxable post-employment notice pay and the amount of the balance of the payment which is above the £30,000 exemption limit. 
Do not include any amount of the payment which is not taxable, either because it is specifically exempted or because it is taxable overseas and not in the UK. 
The amount up to the £30,000 limit goes in box 9
Box 9 along with a worked example is on Page AiN 4
Thank you