Hi, thank you for your question. Initially, you need to formally write her a letter before action - in the letter you need to outline the debt that is outstanding and provide a reasonable deadline for her to settle the debt, and state that if it is not settled by this date that you will pursue a court application for the debt.
If the debt is not settled voluntarily, you should then proceed with a small money claim which can be commenced here: https://www.gov.uk/make-money-claim-online
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