Hi Alisdair,

Asuming that the first column is A , secound column is B and thrid is C
data starting in row 1 you could use Vlookup in column D to retrive the
diagnosis text for column A

=VLOOKUP(A1,$B$1:$C$12490,2)

You can copy this down in column D starting at D1. The column D should
match column A

Regards,
Bondi