For statutory interest, the reference rate is the Bank of England base rate in force on 31 December or 30 June immediately before the six-month period in which the debt became late. That rate then applies for the whole time the debt stays unpaid — even if base rate later changes.
This is why two invoices that went late a few months apart can carry different interest rates.
Example
A debt that became late in August 2026 uses base rate on 30 June 2026 (3.75%), giving statutory interest of 11.75%.