[AccessD] Query to find records from lookup table NOT in another table

Hi All,

I've really got my stupid head on today and can't get anything I try to 
return the records I need.

I have a table which has a list of codes and another table that holds 
details of codes that are in use at various sites.

What I need to find is those sites that DON'T have any of the codes from 
the lookup table.

I thought a find unmatched query would work but it doesn't return the 
correct records.

As an example of what I need

Lookup table has for example
A1
A2
A3

Sites table has
a    A1
a    A2
b    A3
c    A1
c    A2
c    A3

so I need the query to return
a    A3
b    A1
b    A2

Can anyone help on this one  ?

Many thanks in advance,

Chris Swann