This is thought to affect thousands of workers so definitely worth checking.
"The judgment clarifies that all workers on a permanent contract throughout the year, including part-year workers, are entitled to a minimum of 5.6 weeks’ holiday a year, irrespective of how many weeks a year they actually work.
This decision is likely to have a significant impact on the holiday pay of term-time workers, peripatetic music teachers, exam invigilators, sports coaches and other workers in the education sector who work on a casual, zero-hours or part-year basis.
Historically, many schools have been applying the 12.07 per cent method, which was previously set out in Advisory, Conciliation and Arbitration Service (Acas) guidance, to calculate holiday pay for their part-year staff.
....An extreme example of how this might apply is an exam invigilator who works only three weeks a year but who works 40 hours in each of those three weeks. Assuming they are employed under a permanent contract, this part-year worker would be entitled to 5.6 weeks paid leave at their average week’s pay, which would be their weekly pay during the only three weeks they work each year.
This results in holiday pay almost twice the amount of their actual annual earnings."
More details here:
www.tes.com/magazine/leadership/hr/how-schools-should-calculate-holiday-pay-part-year-workers