Access97 Validation dependent on other r

I need to validate an entry depending upon values in another record. In detail a record has a composite PK consisting of a CodeNumber and a Letter (only A,B or C). Valid entries are for example:
CodeNumber Letter
1234 A
1234 B

OR

1234 C

but no other combination of use of the Letters.

I reckon I need some sort of trigger on insert on the table, but Access does have that facility.

Any suggestions will be greatly received.

John