Sub Forms

I Have two Tables. the first holds various information with it's own Primary
Key and also a Supplier ID "SW_Opp" that identifies a Supplier. The second
"Suppliers" Table holds the "SupplierNo" number as the primary key and lists
the "SupplierName" plus other info about the supplier. The first table
"SW_Opp" is the same as the second table "SupplierID" and they are linked in
the Relationship.
I have a Sub Form that has its record source from the First Table, so all
the Text boxes have their control Souce based on that Table. One of the Text
Boxes in that sub form is called "SW_OPp and returns the number of the
supplierlinked to that particular record in the first table. What i want to
do is to have the supplier Name (held in the Suppliers Table) appear in the
sub form depending on the Supplier number held in first table. I thought of
using an unbound List Box and have tried to set the Row Source to get the
result I want.
I have tried many combinations but they all come up with an error. The one
that I thought should work is as follows :-
SELECT tbl_Suppliers.SupplierName FROM tbl_Suppliers WHERE
(((tbl_Suppliers.SupplierNo)=[sub_frm_StoreAddressExtra].[SW_Opp]));

It still comes up with an error and I must be doing something fundamentally
wrong.

Any Help please?

RayC