Got a single state work flow on the PO. When approved an email gets sent out. However, when I add a condition besides approved (AND Created by = X) no joy. Any thoughts?

Try to set that "created by" value in custom WF field and see what log shows afterward. I mean you will know is the value such as you expect.

Thanks Arunas, just before your suggestion I tried that and it seems to work now, strange. You noticed this before previously as well?

no, I didn't face such issue before