I didn't think to link it to the field in the same report. Whoops!
I did this - =IIf(DateDiff("d",[DateDue],Date())<14,"Urgent",IIf(DateDiff("d",[DateDue],Date())<30,"A","B"))
However, it came up with Urgent, even though the date due is 12/10/10, which is obviously more than 14 days.
Thoughts? Eventually I'm going to get really good at this and help people too but for now I'm just confused!