I also found this example
Example
If the base rate is 4% for the six-month period when the debt became late, then the statutory interest rate is 12% (4% base rate plus 8%)
Debt is £851.06 plus £148.94 VAT = total £1,000
If this debt is 30 days late, then the interest owed is:
£ 1,000 x 12% = £120 (the annual rate)
£ 120 ÷ 365 = 32.9p (the daily rate)
32.9 pence x 30 days = £9.86 (the interest owed to date)