Hi Mark,
Calculate your hours, then calculate you minutes.
Hours=DateDiff("h",[DateTimeStart],[DateTimeEnd],0,0)
Mins=DateDiff("n",[DateTimeStart],[DateTimeEnd],0,0)
then you need to take your hours * 60. and take this value away from Mins
ie
Mins =(Mins - (Hours * 60))
Now you should have...