Dear Uncle Gizmo and twoplus two,
Thanks for your help. This expression did the business in the end (I had a ; instead of an , :o).
RfiRef: "RFI" & "-" & [ProjectNo] & "-" & [TaskNo] & "-" & Right([RfiNo]+1000,3) & "-" & Right([RfiIssue]+1000,3)
Ref...