The rate is 8% a year over the Bank of England's Bank Rate. Which Bank Rate counts is fixed by the date the debt became late: the rate on 31 December for interest that starts between 1 January and 30 June, and the rate on 30 June for interest that starts between 1 July and 31 December. For interest starting in the second half of 2026 that is 3.75%, so 11.75% a year.
On top of the interest there is a fixed sum for the cost of recovering the debt, set by section 5A of the Act: 40 pounds on a debt under 1,000 pounds, 70 pounds on a debt of 1,000 pounds up to 9,999.99, and 100 pounds on 10,000 pounds or more.
An invoice of £5,000 due on Wednesday 30 September 2026 and paid 31 days late carries interest at 11.75% a year: £49.90. Add the fixed sum of £70, and you can claim £119.90.