Isn't this becoming worryingly common? Now I happily watched a Citation zoom beneath my LHR inbound in exactly the same place last week but I expected he'd be at least 1000 ft below as I am familiar with the area.
You've got me scratching my head there, all I can think of is given the procedure put in place following this incident, it just goes to show how difficult it is for observers on the sidelines to accurately judge distance.