The passenger who tweeted the picture above, @TBoneGallagher has just been interviewed on the BBC.
Quick notes: 8-9 minutes into flight, there was a loud popping sound. Looked to left window, left side engine cover blown off. Apparently the same happened on the right at the same sort of time.
Captain came on intercom, said they were aware, both engines were functioning normally, they were running tests.
About five minutes after that, loud sound, right engine clearly on fire, flames, smoke visible from cabin.
Plane banked to right, no more severely than normal LHR stacking, flew back to LHR.
The flight was no less smooth than normal.
Cabin crew very professional.