Actually, this most recent case is a helpful guide for what to do when a part-time employee increases their hours part way through the holiday year. The case went all the way to the European Court of Justice who held that annual leave must be calculated in accordance with a worker's contractual working pattern. The taking of leave accumulated in one period has no connection to the working hours in the later period when leave is actually taken.

There is already authority that a reduction from full-time to part-time working should incur no reduction to the amount of leave a worker has already accumulated.

So, the rule is that a worker accumulates holiday based on their working pattern at that time. For example a worker on 2 days per week and entitled to 6 weeks holiday per year will accumulate 2 days per month. At the end of June (ie. half way through the year) they will have accumulated 12 days holiday. If they then change their hours to 4 days per week, they will accumulate 24 days in the second half of the year. By the end of the year they will have accumulated 12 + 24 days = 36 days. It is irrelevant when they take those days off. If by the end of October they have not taken any holiday, then subject to the needs of their employer, they will be entitled to take all 36 days.