Now that the report is published, I'll offer my thoughts.
The report certainly raises some interesting points. To my mind, it also appears not to address a couple of important points. I should declare, at this point, that I have never flown the Gnat, but I do have some 5000 hours of fast jet flying. I have never done, nor ever had the desire to do low level aeros although I have been involved in flight and display supervision.
His experience and currency is obviously the main issue that stands out in this case, followed closely by his undeclared medical condition, which may be irrelevant, but surely deserves consideration, given that he may have had symptoms - occurrences of symptoms of the condition only disappear over time in 25% of cases (Mayo Clinic website).
It appears, from the report that the relevant sequence of events was two rapid aileron rolls, nose drop at low altitude and application of an incorrect recovery technique - for whatever reason.
The two rolls were separated by a short pause, so it seems unlikely that inertia coupling was an issue provided there was no longitudinal stick input. Although, the shorter the pause, the greater the likelihood of residual yaw/pitch from the first roll. Alternatively, it is possible that the pilot has applied back stick to correct any nose drop from the first roll and had not fully removed that input during the entry to the second roll. Any residual rearward stick input would induce a pitch down after the first quarter of the roll - as could air disturbance (jet wash, etc). The situation may have been recoverable at that point.
What follows is based on the report's ascertain that technical faults were not determined, although that does not necessarily mean there weren't any.
Commanding right roll was appropriate as would rearward stick movement once the wings were level or close to it. (unload, roll wings level, pull to the buffet). A hard pull when overbanked was not. Also the degree of aft input looks like it was large enough to stall one or both wings. So the question is why pull at that stage and why so hard? Some possibilities:
Lack of experience in Gnat handling.
Lack of training in the correct recovery technique.
Momentary desperation.
Or, not really addressed in the report, disorientation. Causes? Rapid head movement for some reason during the roll or between the rolls (looking for his number two?), increased susceptibility on the day due to physiological factors or dizziness brought on by his medical condition - is a listed symptom. I doubt that gloc was an issue at that point.
I look forward to comments from the TP, Gnat pilot and QFI fraternity. I'm just a QWI.
I do have to say, it's a good report, but it does leave some gaps in the findings. I also wonder how well a PPL pilot understands the intricacies of swept wing aerodynamics, inertia couple ing and the bit with K, Q, cam and gear in it?