OK, found my own solution.
My Table One has a second report attached to it which had sorted the data.
Table Two, being new, had not sorted the data, so despite SQL being SQL, it still needed a sort based on my required fields to make it work....
Sort, sort, sort...First rule of Filemaker, right?
That doesn't sound right unless your calculated field uses GetSummary, or you mean "found" instead of "sorted".
Address::County will access a value from the County table occurrence. Whether or not you can access a value from the table identified by that table occurrence will depend on the "context" in which your calculation evaluates. If this is a script step this is determined by the layout that is current and the values in match fields of the current record on that layout. If this is a calculation field, the "context" table occurrence that you select for your calculation determines the starting point for determining what, if any, value is accessible via that reference.