Our GM has sold us all a story about how hard up NATS are and how we should not claim AAVAs and put in unpaid duties.
If this is true, your union reps should consider lodging a formal complaint. The GM has no power to override the national AAVA agreement (which includes payments !!).
He might ask people to consider working for 'time off in lieu' but I am not sure that Prospect would be too happy with that for operational duties.