The bailiff is charging interest on the judgment debt.

The county court may award the claimant simple interest on the judgment debt at a rate it thinks fit from the date the money became due, until the date the debt is paid.^[1]

The rate of interest is 8% simple interest a year or any lower rate at the court's discretion.^[2]

In the High Court, the debtor is liable for simple interest at such rate as the court thinks fit.^[3]

Search Trust Online^[4] and get the judgment date and amount then see whether interest has been awarded and at what rate. If no interest is awarded, then the default rate of 8% simple interest applies from the date of judgment for a maximum of six years.^[2]

^{[1] Section 69(1) of the County Courts Act 1984
[2] Section 17 of the Judgments Act 1838
[3] Section 35A(3) of the Senior Courts Act 1981
[4] See: Trust Online, register of judgment maintained by HM Courts and Tribunals Service}