Monarch pay the handling agent, the handling agent pay the airport, whose responsibility it is to maintain the airport infrastructure so I would have thought. As to compo I cannot answer that, let those in the know have a say.
Every passenger who was delayed over 180 minutes (outgoing or incoming) on either of the two days due to the computer failure is entitled to 261/2004 compensation.
As you would expect (due to the extent of the problem) MON are, at the moment, refusing all claims indicating that the problem was outside their control.