2017/18 Tax Return filed late (17 April 2019) showing a liability of £3K
2018/19 Tax Return filed on 24 May 2019 showing a refund of £3.3K. Said refund has been offset against above liability.
I was expecting HMRC to charge interest on the 17/18 liability from 31 January 2019 until 23 May 2019, but instead they have done so up to 30 January 2020. HMRC advise that this is because the effective date of payment for the refund is 31 January 2020, rather than the date the 18/19 Return is filed.
This seems grossly unfair as for 8 months or so they are charging interest on a debt that physically does not exist. But I cannot find anywhere within the Self-Assessment or Debt Management manuals which suggest either of us is correct.