Can anyone advise me how to create an IF command in an Excel data
source which will enable any letter created in word to include a
sentence if the Postcode entered is included in the 'IF' cell.

To clarify I need to input a sentence into a word document which has
data entered via mail merge but only if the postode is equal to any
one of three codes.

Maybe it would work to have an "IF" column, where each cell evaluates to
either an empty string or the desired sentence. Then include this as
another merge field.