Interest is not charged on late returned self-assessment returns, but rather on any tax shown as due on a return and unpaid at the due date.
HM Revenue and Customs recalculates its interest rates following any change in the Bank of England base rate in line with procedures laid down in legislation. The legislation sets out how the rates for various taxes and duties should be calculated and when they apply from. In accordance with the legislation the rate of interest charged on underpaid income tax is currently 3.5 per cent. This rate has applied since 27 January 2009.