If the cake was unsatisfactory, to the point where it was inedible, then surely it would all be left!
There is no point in hanging around, I would tell her that either she pays the balance by 4 PM this Friday, 18 September 2020, failing which you will issue Small Claims Court proceedings without further notice.
www.moneyclaim.gov.uk is quick and easy. You can prepare the proceedings now, and save them online ready to issue them whenever a deadline for payment (you have to give them a deadline in writing and it has to be reasonable period to pay up) expires.
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