Brilliant! Thanks so much. I can add this on several pages.
The other instance is in an insert behaviour that tracks visits to the page; if there is no referrer, I get a similar error about this line:
$InsertQuery->bindColumn("st_referrer", "s", "".$_SERVER['HTTP_REFERER'] ."", "WA_DEFAULT");
Where would the isset go in this example? (I have tried, but no success)