Thanks for all the replies.
We didn't let it spoil our holiday, and they got us home on time at least.
Having said that, my seat on our flight home was the most uncomfortable I've encountered on a plane for years. I can understand them putting seats close together with little leg-room, and making them narrow, all so they can pack more seats on the plane and offer cheaper fares. What I don't understand is what they would gain by making the seat back so short that it doesn't even come up to my shoulder! I may be taller than average, but I'm not freakishly so, and I noticed others in the same predicament. If the flight was only half an hour or something, it wouldn't be an issue, but on a flight departing at 2.20 in the morning and lasting the best part of 4 hours, I needed somewhere to rest my head. I can only assume that they expect tall people to recline their seats and rest their heads in the laps of the people behind them!
I'm well used to the shortcomings and disadvantages of flying with low cost and no-frills airlines, and take most of these things in my stride, but I think it'll be a while before I'll be open to being persuaded to fly with Monarch again.