The a/c at LTN did not go tech.
The TOM B737 was subbed in because if they waited, it would not have been until late tonight when a returning a/c from its second rotation of the day would become available.
MON knew yesterday that one of their a/c (A320) required an engine change found during its routine hangar check and would not be completed for this mornings flight.
That's the difference between Monarch and the likes of BA, Easyjet and Ryanair, they would more than likely cancel the flight, where MON will sub in.