>Hi thanks but I don't understand, what I'm trying to do is nothing more than create a one to many relation which I've successfuly done with other tables, e.g. I have a master table with a pk of MasterRef and a Claim table with a fk of MasterRef and I managed to create a reference without a problem and the claim table has many MasterRef values which are not unique

In your case it looks like you reversed the relations.

In Address table AddressID should be a primary key (most likely clustered) and in the Customer table it should be a foreign key referencing Address table.