From the usual excellent reliable news source (AvHerald):
On May 29th the NTSB reported quoting the AAIB, that both engine cowl doors separated during takeoff and fell onto the runway, one engine was leaking fuel and had been shut down, they were returning to Heathrow. Later the crew reported the other (still running) engine was on fire, the aircraft continued for a safe landing, was shut down and evacuated. The NTSB assigned an accredited representative into the investigation led by the AAIB.
Several aspects of that report sound highly dubious.
Firstly, it would be a very unusual breach of etiquette for the NTSB, in assisting an AAIB investigation, to release information that
AFAIK the AAIB itself has not yet placed in the public domain.
Secondly, we know from the video showing the thrust reverser deployment that the aircraft landed with No1 engine operating. If it, too, was on fire then it's strange that none of the video or eyewitness evidence supports this claim. In fact, AvHerald also quotes a passenger on board as saying:
"I was a passenger on the flight sitting next to engine that caught fire. Facts are that covers (cowls?) on both engines fell off at take off and hit the airplane. Liquid started leaking as we climbed further. Took 10-15 minutes before the captain announced return. As descending the fire broke out in the right engine - quite a blaze at first - burned maybe 30 sec to 1 minute - then it died down as the right engine got cut off. Aircraft swayed a bit before it continued in a stable descent. Worried about the left engine as it was fully exposed as well. Landed maybe 5 minutes+ later pretty normal with 1 engine running"