So as I understand it, accrued leave is basically leave in advance, which employers are allowed to do. For closedown periods, it says employers may allow employees without enough leave to use leave in advance, otherwise they take it without pay (see here).

You can't force your employer to let you work if you don't have enough leave, you just either take leave in advance or take it unpaid. It's not clear whether your employer can force you to take leave in advance instead of you deciding to take it unpaid so you can use your leave later.