I'm hoping this is the final change to the AppleScript 
(Sorry for the clutter! Please let me know if I should remove the old versions from the posts above. I wasn't sure whether it was helpful to leave old copies around so that people could see the evolution...)
It now uses an AppleScript 'where' clause to find the substitution (rather than iterate over the table), and selects the row (as well as focusing on the 'With' value), and also bounces the cursor to trigger a scroll to that row ... all of which seem to have improved reliability (previously, the update was sometimes rejected/ignored).
It's 2021 and I didn't expect to still be writing (and learning a lot more about) AppleScript and UI scripting, but here we are 
