Thanks for both of those tips, I will do both.
Interestingly I found a different wrinkle on this as well, if you take the expression:
=Sum(IIf([binAuthorised]=-1 And [binCancel]=0 And [fkAbsenceCodeID]=[Forms]![frmPersonnel]![frmAbsence].Form![cmbAbsenceCode].[Column](0),[numDays],0)
and...