You can use this Letter if there is no contract between the debtor and yourself. However, if there was no contract and you wish to charge interest, you can only claim statutory interest. This only applies to business-to-business transactions. Currently, the statutory interest is 8% plus the Bank of England base rate for business-to-business transactions. You cannot claim statutory interest if your customer is a consumer and not acting in the course of business. In such scenarios, you can only claim interest if your contract sets an interest rate.
For more information, read Calculating interest on commercial debts.