This web-site includes a calculator for working out the amount of interest that can be claimed. It can also work out the amount of compensation that can be claimed. If you are intending to take legal action to reclaim the money, the calculator also provides you with suitable wording for your claim. [Calculator updated to include interest rates - up to July 2014.]
To use the calculator you will need to know the date on which the payment was due, date paid (if debt has been paid) and the amount of the debt.
Date due: The date by which the debt should have been paid. Typically this might be 30 days after the invoice date.
Amount of debt: This is the amount of the debt that is still outstanding (or was paid late).
Date paid: The date on which the debt was paid, or if still outstanding use the current date.