It could be that Martinair have got an allocation of seats on the First Choice flight…it often happens to save operating two separate flights to the same destination!

I am not convinced of that. Martinair is in stiff competition with ArkeFly, the Dutch brand of the TUI empire. It would surprise me a lot to see TUI handing over business to Martinair, even if it TUI where to make money from it. My best would be that you’ll fly to AMS and then catch a Martinair flight.