When a monitor exits maintenance mode its state gets set to "uninitialized" (no history). In this case it would take the regular amount of time to trigger alerts, but it should do so. By default all alert related history is recorded in the logs, it's handy to go back and check on if alerts were triggered or skipped.
Thanks for your help.
Turned out to be a classic case of Catch 22. The reason we didn't get any email alerts is because the monitor that was down was our email system(!)
Time to configure up some more alerts on our new GSM modem.