My formula has magically stopped working.. it used to look like this:
=IFERROR(IF(COUNTIF(BookData!O:O,EnqData!Q3)=0,"",INDEX(BookData!B:B,MATCH(Q3,IF(BookData!H:H>=C3,BookData!O:O,""),0),0)),"")
What this does:
First portion - Counts whether the unique customer reference in Q3 matches...