As
@Mottie et al have suggested, tell her that you will lodge a small claims case. In some respects you will both lose out- both of you will need to take time off work to sit before the magistrate (which is not refundable). If she doesn't turn up to court, it is very probable that the judge will automatically find in your favour. From memory, the case will be heard in
your nearest county court. If she loses and does not pay within a month, the CCJ will sit on file for 6 years and is likely to impact her credit rating
https://www.gov.uk/county-court-judgments-ccj-for-debt/ccjs-and-your-credit-rating.
The magistrate will want to see that you tried to correct the situation before bringing the case to court. Thus far,it sounds like you have tried (up to this point).