I can imagine that had the work at Derby shown there to be an issue with the engines' fuel systems, then that would have been reflected in today's AAIB update. But... apparently not. So, it looks as if summats up with the aircraft's fuel system, for whatever reason.
What - maybe something like:
Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected.
NoD