All businesses have a legal right to claim interest from late-paying business customers, but not individuals. The statutory right to interest applies to all contracts agreed after 7 August 2002 and gives you the right to charge interest at the Bank of England base rate plus 8%. The base rate for the current period to 31 December 2009 is 0.5%, so you can charge interest at 8.5%.
The statutory right to charge late-paying business customers interest applies to contracts which do not already include their own arrangements for 'substantial' interest. If there is no agreed credit period in your terms of business, the law sets a default period of 30 days. You can charge interest 30 days after you delivered the goods or provided the service, or 30 days after you notified the purchaser of the amount of the debt - whichever is the later.
Ideally, to notify the purchaser of the amount of the debt, you should send an invoice that includes details of the credit period but any other form of notification would do, including a phone call - though that might be difficult to prove if there is a dispute.
All organisations also have a right to claim reasonable debt-recovery costs. Creditors can claim an extra £40 for debts of up to £1,000, £70 for debts from £1,000 up to £10,000 and £100 for debts of £10,000 or more.