Creating A Form Lookup From 2 Tables

Hey all.

I'm having a few issues with creating a Lookup on an Access form. I'll recreate the scenario below as best i can to help with my explanation.

I have 2 data entries. The first is a Post Code (Zip code), and the second is an Area Rating that applies to the Post code. For example:

Post code AB1 8 is awarded a rating of A
Post code SO1 3 is awarded a rating of F

I have already created a table in Excel with all 2,922 Post Codes (AB1 1 to ZE3 9) and the corresponding Area Rates (A to F)

What i'm making is a form that holds customer details. I'd like a combo box or text box that when you type in the Post Code, another text box automatically displays it's corresponding Area Rating.

Code:Post Code: Area Rating:------------- --------------| CM2 7 | | A |------------- --------------

I apologise for my really very poor diagram :P

So the Post Code box is either typed into or a combo box with the list of Post Codes, and the Area Rate box cannot be typed in to, but it automatically displays the record dependant on what is in the Post Code box.

Ideally, these will also be recorded onto the main table (IE not the table with the list of post codes or area ratings, but a 3rd table holding the rest of the details that will be used to create statistics and graphs)

Can anyone help me with how to do this? I've been trying to work it out all afternoon!

Any help is really appreciated.