I don't know Phil,
BASSA's best move would be to do nothing, in my opinion. Unite wouldn't act against that, but the membership might. There is the current terms and conditions and through custom and practice, the requirement to work. If you are limited to 900 flight hours and 2500? working hours, then you could have an implied unlawful deduction from wages.
Just a thought, with no basis in case law to back it up.