I've found this:
=INDEX($A$1:$B$7,SMALL(IF($A$1:$A$7=$A$10,ROW($A$1:$A$7)),ROW(1:1)),2)
Which looks at a small table from A1 to B7, looks for a value entered in A10 through A1-A7 and returns the related B value (and repeated versions of this formula will reutrn 2nd, 3rd, xth, related values). But I have to admit, I'm struggling to decipher how that language translates to this.
If anyone could split that into chunks and try to explain to me (I've not used array formulae before, but I'm comfortable with things like VLOOKUP and COUNTIFS) I'd be eternally grateful!