Shame on Monarch!
I agree with SimonS1, take them to court using the small claims procedure. They are often settled before they get to court or often go undefended, so you win outright. Be careful as the fees have increased if you lose but read all the steps on Moneysavingexpert. You can even fill in the forms and apply on line now, so it can be really easy.
I suggest you start now though, as the event has to have happened in the past 6 years (5 in Scotland), so time is running out. The procedure is easy but may not be quick.
Good luck and don’t give up (apart from giving up flying Monarch, of course).