I Don't Want To Speculate
First, the main message must be sympathy for the pilot and his family and friends. Second, the 'experts' effect just won't go away, will it? Thank goodness Sean M fronted up (even though he's a navigator, for heaven's sake!) to put a sensible and informed view. (Greetings, Sean, not seen you since Linton).
Third - and for what it's worth - although I enjoyed the Gnat it had a poor reputation when I was training on it in the 1960s, with eleven serious incidents on my course of 12 students. It was quite unforgiving in some ways, with one of the nastier habits a massive change in feel following immediately after a hydraulics failure, amounting to what felt initially like seized controls, until you ran through the IAs. Not saying that's what happened here, though the symptoms are consistent with the tiny clip I've seen on TV.