I have the following formulae

=IF(OR([@State]="Closed",[@State]="Resolved"), IF(AND([Close Code]="Duplicate Request",LEFT([Short Description],3)<>"ESB",OR([Close Code]="Mistake/raised in error")),"NC","C"))

Basically if state is closed or resolved then

If Close Code

=Duplicate Request AND Short desc <> ESB

OR

Close Code ="Mistake.."

Then enter NC (in either event)

On all other occasions enter C

However, it just seems to enter C on every occasion and i've spent several hours rewriting and trying to fathom out why!!