As a frequent flyer, as a passenger, on both Airbus and Boeing of a similar size, I am amazed by the angels on a pin discussion on this complete cockup.
It is purely through chance that this aircraft returned safely to Heathrow.
Everything from cowling Engineer checks to the walk around by the Pilot appears to be a failure of SOPs and good practice.
Even the return of the aircraft to Heathrow over large populated areas rather than to peripheral airports with low population density appears to have been unwise.
Negligence abounds in this sad and sorry tale.
Luck won out.
The only saving fact is that the aircraft survived this Fawlty Towers flight, and lessons can be learnt.
Implementation of failsafe , simple SOP's and checks seems in order, with appropriate penalties for failure to do so.