In reply to Chris_covert:
Stage 1
Send a letter (keep a copy) saying that there seems to have been a misunderstanding, state that she has NOT had the last payment, the amount due and that she would appreciate immediate payment. Put a deadline for payment (a week maybe?)
Stage 2
Send a letter reminding them of the above and adding that she will persue the case + costs + interest. Keep a copy and send a copy of both to head office if there is one. Again put a deadline.
Stage 3
Fill in the court forms here
https://www.gov.uk/make-court-claim-for-money/overview
Hopefully Stage 1 or 2 will work, but if not then that is the option.
Some example letters here
http://www.qualitysolicitors.com/small-claims/pay-or-wage-disputePost edited at 18:34