The Late Payment Act lets a supplier claim a fixed amount for the cost of recovering a late B2B debt: £40 for debts under £1,000, £70 for £1,000 to £9,999.99, and £100 for £10,000 or more. It is per invoice, not per customer.
If your actual reasonable recovery costs are higher (for example, a debt collection agency fee), you can claim the difference as well.
Example
Three late invoices of £500 each can each attract £40, so £120 in total.