January tax payment question

[Panamax]

Any knowledgeable thoughts on this one?

• Self Assessment income tax return filed online May 2021
• HMRC's immediate calculation correctly showed tax refund of £5k was due from HMRC
• June 2021, HMRC embark on a detailed identity check requiring to see passport, bills with address etc, etc
• Nov 2021, HMRC letter says identity checked out OK but they're not happy about the tax return. No details provided.
• Dec 2021, HMRC write saying they have "corrected" the tax return and £20k tax is due in Jan 2022 plus a payment on account of £10k. Total £30k
• Immediately wrote to HMRC rejecting their "correction" and pressing for the original and correct tax refund of £5k.

Question: Assuming HMRC move at their usual s-l-o-w pace and things don't get resolved in the next few weeks should I,
Do nothing and just wait, because their tax assessment is obviously wrong? or
Pay them £30k and then argue to get it back again, plus the £5k refund they owe?

Whilst it's easy to spot which I prefer it would be unfortunate to get into trouble if that's a potential outcome of not paying.

[Eric Mc]

What have they done wrong to arrive at a wildly different tax situation to the one you calculated?

Is the Self Employed Income Support Scheme involved?

[Panamax]

Thanks EricMC for your response. It's actually a tax credit they've suddenly decided not to believe. Sent them some "proof" in mid-December and hopefully they'll straighten everything out by the end of this month. I'm simply wondering about the best way forward if they don't respond before 31 January.