MUFC Fan
Operations departments don't "swing" aircraft availability.
Commercial departments work with fleet planning to look at what availability they can free up, based on seat sales (weeks/months in advance) and knowledge of potential requirements based on history.
It was infact 6 individual 757s and 5 individual 737s which operated services to HAM. 2 757s were required to return to the Uk to operate services whereas the rest daystopped/nightstopped downroute.