Discrepancy between self assessment overpayment letter and online account balance

I've recieved a letter (dated 19.07.23) stating that I have overpaid my tax by a substantal amount, for the period 2019-2020. It states that if I have claimed a repayment, this will be repaid after any other amounts I owe HMRC have been cleared. My online account states that I am owed what amounts to less than a resturaunt tip and I have no way of reconciling this discrepancy or finding out why it exists. I therefore can't claim the overpayment refund quoted in the letter because it is not reflected in my online account. The digital chat on the self assessment web page automatically refers to tax credits, rendering it useless and the telephone helpline has been 'suspended' until September 2023. How do I make any progress beyond my current catch 22 situation? Why is HMRC so hard to make contact with? Where are the humans?

Hi
If you use 'Ask HMRC Online' and state you wish to speak with an adviser you will be put into the queue for Webchat and we can look into this for you.
Self Assessment: general enquiries