You have a claim under EU regulation 261/2004 and you should write and demand compensation under this provision. Your flight was cancelled. Your rights are outlined here..
http://en.wikipedia.org/wiki/Regulat...g_or_refunding
It is not easy for an airline to rustle up another aircraft at short notice, nor is it straightforward to "swap one plane for another". However, there is no excuse for the sheer incompetence and lack of communication shown here. You can pursue your rights through the small claims court, if Monarch don't play ball. It is easy, can be done online and is relatively cheap.
The link is here:
https://www.moneyclaim.gov.uk/web/mcol/welcome
Good luck.