The contract is fine but a worker can cancel the opt out if they wished (in writing, giving I think 7 days notice) and (in theory at least) they can't be discriminated against for doing so.

Worth noting too that the average of 48 hours a week is calculated over 17 weeks so under even under a WTD compliant agreement you could have someone work a month of 60 hour weeks so long as you reverted to 40 hour weeks for a while after to bring the average back down.

The problem here is that any piece of legislation in this category with a "in theory at least" clause against discrimination will become meaningless. This type of legislation must be mandatory, or at the very least subjected to individual exemptions based on centrally negotiated agreements.