I'mnot sure if it is the same as the saloon but I'm guessing so.
If you lean into the boot and look back at your feet you should see two nuts in the back panel. Sometimes these are covered in a putty/tar like substance if they've not been removed since new. There are then two clips on the sides. You have to remove a black plastic clip from inside a plastic box, and then the bumper simply comes off backwards.
If you get stuck with anything else then try asking on the maestro and montego owners club forum ( www.maestro.org.uk). The details might prove handy for the guy at work.