"What reason do you have to distrust the PA announcement about an engine problem ?"
Agreed - I was on an Air Canada 747 out of LHR late 80's where exactly that happened - the plane sort "wallowed" 2 hours out and then a very wide turn and back to LHR - we sat on the aircraft for about 90 minutes while they fixed something on the starboard outer and then took off for a successful trip to Toronto