Hi Matt Creaney
Just as a test, replace the whole expression with just:
return $feature["Asset_Name"]
and click on the blue "Test" button.
That should give you the asset name of a feature.
Then modify this line to show:
return Find (XXXX, $feature["Asset_Name"], 0 ) where XXXX is the value returned from the first test.
Does it return -1, 0 or some other value?