When it comes to air safety there are no "Unless" issues apart from, in this case, something within the military's own organisation - which is the point I was making.
As no airworthiness issues are identified in the civil environment, the problem seems to lie within the BBMF alone. If it were otherwise it would (or should be) a matter that needs to be shared with all operators of either Spitfire and Hurricane aircraft or Merlin engines, as well as other non-EASA "Permit to Fly" types that have Merlin engines installed. As the grounding affects different aircraft types and different marks of the Merlin engine, I suspect oil or fuel system contamination or something similar as a most likely scenario.
Last edited by Blacksheep; 18th Aug 2017 at 14:33.
Reason: To add a root cause opinion