The interest you can charge if another business is late paying for goods or a service is ‘statutory interest’ - this is 8% plus the Bank of England base rate for business to business transactions. You cannot claim statutory interest if there’s a different rate of interest in a contract.
You cannot use a lower interest rate if you have a contract with public authorities.
Check the current Bank of England base rate and previous rates.
Example
If your business were owed £1,000 and the Bank of England base rate were 0.5%:
the annual statutory interest on this would be £85 (1,000 x 0.085 = £85)
divide £85 by 365 to get the daily interest: 23p a day (85 / 365 = 0.23)
after 50 days this would be £11.50 (50 x 0.23 = 11.50)
Send a new invoice if you decide to add interest to the money you’re owed.