G'day Rob

You could try something like:

Either in WorkBook1 (somewhere off screen), or in a NewWorkSheet, you could
setup an Array.

Assumed NewWorkBook
A B
ABC 123
BCD 234
CDE 345
DEF 456
EFG 567
FGH 678
GHI 789

In WorkBook1

B1 =

=IF('WorkBook2'$A1="","",(LOOKUP(WorkBook2!$A1,New WorkBook!$A$1:$B$7,NewWorkBook!$B$1:$B$7)))

Copy Down as required

HTH
Mark.