Ex RR Mech Eng here (a long timer ago). My gut says (with no real facts yet) this is most likely a disk (or maybe compressor blade) metallurgical problem that was missed in ultrasound/x-ray testing during manufacture. Since these critical parts are tracked from the beginning of the manufacturing process it should be possible to trace it back almost to the plant that made the original ingot. Hope this is the case for the guys in Derby since this would be better than a design defect (which would surprise me).