It says in this article,
"In the latest incident, a No 436 bus was gutted after flames were seen pouring out of the engine at the back of the vehicle as it travelled past the Hilton Hotel on Park Lane.
Passengers were evacuated before a fireball engulfed the entire length of the 60ft bus on Saturday evening.
Within a matter of minutes it was left a burned-out shell with only the metal chassis remaining.
A fire service spokesman said: "There is obviously a mechanical fault there."
later in the article,
"A TfL spokesman said:....But he said the buses would not be withdrawn."