You can recover the money from them the same way you would from anyone else.
Write a formal letter to them stating the money owed and that you require the agreed repayment within a reasonable time, you can specify that you are starting to charge interest on the amount outstanding and will recover the money by issuing a claim if necessary. If repayment terms were agreed then plainly you may only request such sum in accordance with those terms.
If you do not receive payment you can issue a claim for yourself through www.moneyclaim.gov.uk
It's very straightforward and cheap.
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