Mixed. The old TOC our annual leave was divided into adhoc (pick a day, book with rosters, see if available and hope...) and block leave (four weeks, taken as whole weeks off) organised on a rota by the union. However we had the option to ask for swaps and changes in a month and the union reps would alter the block leave roat accordingly. You want to move your Spring week to an empty slot? Not a problem, you got it. Someone else is off? Will they swap? Sometimes it got to an informal messroom conference.