Depending on Monarch's scheduling agreement, the duty would work with a split allowed under CAP 371.
Report 1600z = Max FDP based on 2 sectors of 12hr 25min
Depart 1700z
Arrive 1920z
Off Duty 1950z
Report 0500z = Rest of 9hr 10min
Depart 0600z
Arrive 0830z = Total Duty 16hr 30min
Last on chocks = 16:00+12:25+4:35 (Half of 9:10)-24:00 (To covert back to a time since over midnight) = 09:00z
As long as the crew get their required rest downroute, and the aircraft arrives back on stand by 0900z then the flight would be legal if Monarch schedule up to max FDP allowed under CAP 371... Although I suspect they may not.
The arrival into MAD will be at 20,20 as there are an hour ahead.