FrustratedTaxPayer

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.