Unfortunately, many of these problems are caused by the handling agent downroute who frequently misunderstand communictaions that are sent from head office. Do you honestly believe Monarch wish to fly an A300 back to blighty with 18 pax.
Its the handling agent that is checking in the pax. A quick check on the computer and they can see that the flight is currently checked in with 18 pax, not FULL.
I sure Monarch will give them a right rollocking once this has blown over.