So your employer basically knows that they can sh*ft you big time - you work for them every day, incl. weekends, I suppose you are on call while on holiday as well. There does not appear to be any pecuniary reward for that, and time off in lieu you don't take. In short, you act like you own the place (by that I mean that any cost saving meams profit for you), even though you receive a salary.

You do this because you (a) like your job, (b) can't find another job if you left the one you currently have, or (c) a combination of (a) and (b). So what leverage do you have over your employer? You're not leaving even though you're working 7 days a week - you're not going to leave because of Y seats to MEL.