Overpaid or underpaid tax

hb1

Hoping someone can advise.
On 13th Feb 2020, we applied for marriage tax allowance, realised that soon after it wasn't for us so cancelled it again on 25th Feb 2020. Received an email confirming it had been cancelled.  Since then, we have started receiving contradictory letters from HMRC. They'll say we are owed a refund, next they'll send a letter saying we've underpaid tax and want the money back. After several of these letters arrived on the same day, all for different amounts either overpaid or underpaid, I rang the tax office (July this year) 3 different advisors either cut me off or couldn't be bothered to help to the point of being quite rude.  I discovered that they hadn't in fact cancelled the marriage tax allowance! They took the opinion of that's just tough - just pay what you owe.  Our tax codes looked 'normal', the standard tax code for the year, it gave no indication that we were paying the wrong amount or that the tax allowance was still active. How do I know what, if anything, I owe when the letters all say something different.  Anyway, a 4th advisor came to the phone, much more helpful, took the time to go through everything, then said actually, we don't owe anything and said he would issue a letter confirming this.  He was as good as his word, even refunded £20 that they said they owed me.  Fast forward to October 2021, and another demand for underpaid tax - £500! I am at my wits end.  Can anyone suggest how I get this resolved once and for all? I've written a rather angry letter from my husband and I, asking them to look into this properly, not just relying on a computer to keep churning out letters constantly with conflicting information.  Any advice would be appreciated.

chrisbur

In order to understand what is happening here more information is needed.  To start with if you could give the details of the calculation HMRC used to arrive at the £500 under-payment figure.