Thanks Jim.

I think it's slightly clumsy, but using your way of doing it I've come up
with a solution:

=COUNTIF($A$1:$C$10,A1)

So long as column B doesn't contain any of the data I'm trying to count in
columns A or C I think I should be okay.

If anyone can come up with a more efficient way of doing it though, I'd
really appreciate it!