Answer
You can charge interest on an outstanding debt either in accordance with your contract or at the statutory rate.
If you do not have a contractual rate of interest, the Late Payment of Commercial Debts (Interest) Act 1998 provides the basis upon which interest can be charged.
The Late Payment of Commercial Debts (Interest) Act 1998 relates to business debts only and allows you to charge interest at 8% above the Bank of England base rate.
If your debtor is an individual, the County Courts Act 1984 allow you to claim interest at the statutory rate of 8% per annum.