Oh dear. You're not going to like this I'm afraid!
Having flown with Monarch for the first time to Grenada in November/December, we would NEVER ever fly with them again. Even if it costs considerably more to fly Virgin/BA/whoever, in our opinion it's definitely worth it.
I've flown quite a bit & I can honestly say that, apart from very short haul flights (& even then I'm not sure they were as bad), I've never been so cramped. & I'm slim & only 5' 5". OH, who's six foot, was in pain by the time we arrived.
There was also a bunch of drunken lads on our flight (both ways) who were incredibly loud (tototally harmless though thankfully). Masses of people complained but the flight crew seemed more intent on chatting them up - I gather, from listening to them on the flight back, they did all meet up at some stage in Grenada! Actually, it's just come back to me: one of them swore constantly & if I'd had kids with me, I'd have been livid. Several passengers resorted to asking the lads directly to keep it down but the crew did absolutely nothing.
So, all in all, one of the worst flights we've had. Sorry about that .... I'm sure you wouldn't have the same problem with other passengers but the space is absolutely appalling for a long-haul flight. It'd be worth paying the extra whatever it is - £75 each I think - to get the seats with more leg-room.
Hope you have a lovely time regardless.