OK, I got it.
If any other noobs are interested:
The drop-down list pulls in a list of values from the first column (Change Number) of a Sharepoint list.
I've inserted a Repeating Table next to it that is bound to the second column of the list (Change Title).
I've used Conditional formatting to:
- If the drop-down selection-box is blank, hide the table.
- If the value of the drop-down selection-box is not equal to the value of the 1st column of the Sharepoint list, hide the table.
and hay presto! When a user selects the Change Number, the correct Change Title is displayed.