Setting User "Role" using Custom Registration

I have a custom field that I have added to the Registration form and this does function ok.

However I also want the form to update the wp-admin user “Role” field to what is selected in the drop down box.
Data coming back is correct ie If the user selects “Basic” from the drop down list, it returns “s2Member Level 1” (which is what I want.

Problem I have is what do I call the “Unique ID” field in the custom field so that it applies this data to the wp-admin “Role” field.

The custom field is adding a second field also named “Role” in the wp-admin user info.