This is quite interesting; I wonder if there was an aircraft substitution. On 3 Oct (When the flight must have departed-- by my calculations, UA 904 departing on 4 Oct can't have reached LHR yet), I flew to SFO on the last flight from JFK and at the time JFK-LHR 956 was being delayed. Looking out the window of my plane at 956, I saw a couple maint. folks on a ladder with flashflights looking into the starboard engine.