Holiday that follows the contract, not a number in a box
A kitchen porter on twenty hours and a supervisor on a salary do not build up holiday the same way. When the contract changes, the entitlement should change with it — worked out from the terms in force, not typed in once and forgotten.
Holiday is where a lot of small hospitality employers quietly get it wrong, and not out of meanness. Someone types "28 days" into a box when a person is hired, the person's hours change three times over a year, and the box never moves. The number on the screen and the number the law actually owes them drift apart, and nobody notices until it is a leaving-day argument.
The fix is to stop treating entitlement as a number and start treating it as a consequence of the contract. A contract is a dated record: it says, from this day, this role, these hours, this pay basis. Store it that way and the holiday follows — accrued from the hours actually worked under the terms actually in force, not from a figure frozen on the day someone was taken on.
The rules that bite are the boring ones
The parts that catch people out are the waiting period before leave can be taken, the notice a request needs, the clash when two people book the same week, and — the one that costs money — costing a booked day in hours at the time it is requested, not at some flat daily rate. Get the accrual right and still bill a half-day wrong and you are back to the leaving-day argument.
Why "it follows the contract" is the whole point
When someone moves from twenty hours to thirty, or off hourly onto a salary, a new contract comes into force with a date. Everything that reads their pay and their leave reads the contract in force on the day it is asking about — so a raise next month does not silently rewrite last month, and holiday accrued under the old hours stays accrued under the old hours. The record is the source of truth, and the number on the screen is just what the record currently says.