Error in SA tax calculation

I've recently had to go through my past HMRC self assessment tax calculations (for unrelated reasons). While doing so I discovered what I believe was a mistake by HMRC in my 2011/12 and 2012/13 calculations (my personal allowance was not deducted from my employment income, resulting in overpayment of tax). I appreciate that you can't give personal advice but can you advise whether it is possible to get this error corrected and if so what action I should take? Thank you.

Is there any possibility you were affected by high income reduction of the personal allowance (to zero) in those years - either obviously or non-obviously through interactions with the calculation of top slicing relief on an investment bond? I'm not saying HMRC couldn't make a mistake, but the personal allowance would be an incredibly routine thing to just forget about... If HMRC did truly make a mistake, those tax years are so long ago, they could probably deny a claim based on it being out of time. I don't have any specialist knowledge about that beyond what I just learned via Google, but it doesn't look particularly promising.

Thanks for your reply. I agree it seems an extraordinary mistake to make and I also wondered whether there was some explanation I wasn't aware of. I was claiming tax relief on personal pension contributions, for example. But I've looked at the tax calculations for several tax years before and after those two years, when my financial circumstances were almost identical, and the personal allowance was clearly deducted. So I do believe HMRC got this wrong and I would like the opportunity to take this up with them, even though it was a long time ago.

Not HMRC...HMRC personnel have zero input in to the calculation, its an automatic computer generated calculated based on information either via PAYE system or based on SA return...to my knowledge its not possible for a member of HMRC staff to remove the personal allowance in a calculation...also, you can only go back 4 years to make a claim, though legislation exists for extended periods IF THERE IS AN OBVIOUS ERROR by HMRC...best bet is to call them (though they may not have immediate access to those years on their system and it may involve a call back).

Hi,
Write to us with copies of your 2011 to 2012 and 2012 to 2013 calculations at:
Self Assessment
HM Revenue and Customs,
BX9 1AS
We can then confirm if personal allowance has been and should have been applied to your income, and what we can do next.
Thank you.

Thank you very much. I will write as you suggest.