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GP practices can levy interest on the late payments of debt. This is enshrined in law in the "Late Payment of Commercial Debts Act" 1998. http://www.legislation.gov.uk/ukpga/1998/20
A payment is deemed as late if it greater than 30 days after either receipt of the invoice or delivery of the service.
Interest is calculated at 8% above the Bank of England base rate.
You can also charge a fixed sum on top of the interest, the value of which depends upon the value of the debt.
A brief summary can be found here -