I’ve discovered a bug in the Due date calculation (in the Maintenance Requirement database) 
Not only might it not work if the interval period was greater than 12, it would also not work if the interval was 12 months and the date of the last successful event fell in a December(!)
Sorry!
Please replace the Due date formula with the following:
Date(Year([Date of last (successful) event]) + RoundDown(((Month([Date of last (successful) event]) + Interval.Period) - 1) / 12,0),(Month([Date of last (successful) event]) + Interval.Period) - (12 * RoundDown(((Month([Date of last (successful) event]) + Interval.Period) - 1) / 12,0)),Least(Day(Date(Year([Date of last (successful) event]) + RoundDown(((Month([Date of last (successful) event]) + Interval.Period) - 1) / 12,0),((Month([Date of last (successful) event]) + Interval.Period) + 1) - (12 * RoundDown((Month([Date of last (successful) event]) + Interval.Period) / 12,0)),1) - Days(1)),Day([Date of last (successful) event])))
This applies to anyone who has already installed it.
Sorry for the mistake.
Let me know if you encounter any more odd behaviours(!)