Your statutory right to interest
Under the Late Payment of Commercial Debts (Interest) Act 1998, interest runs automatically on a qualifying business-to-business debt once the agreed payment date passes — you do not need a clause in your contract, because the Act implies the right. The statutory rate is 8% per annum above the Bank of England base rate. With the base rate at 3.75% as of June 2026, the current statutory rate is 11.75% per annum, as simple (not compound) interest from the day after the debt falls due. Check the current base rate, as it changes.