I think it unlikely that its a hyd fail as the accumulators have enough energy in them to give 3 1/2 full aileron or elevator deflections before loss of pressure
Back from under the rock for a breather. In the meantime, the Hunter notes also show:
With some types of hydraulic failure immediate reversion to Manual will result.
It goes on to say that post mod 452 (which I assume is incorporated):
When mod. 452 is embodied a revised wiring and switching arrangement is applied to the aileron and elevator pawl release units so that Manual is automatically selected if any pawl disengages even though momentarily. The aircraft can therefore be flown only with Power correctly engaged or in Manual
I don't claim for one moment to remember the implications of that but it would suggest that post mod 452 it is an all or nothing system i.e. you're powered or you're in manual, although I'm probably reading it incorrectly and will bow to superior knowledge.
I couldn't find out the hydraulic pressure required for a warning but it does say that below 200 psi the system switches automatically to manual.
Maybe with OAP's vapour being a hydraulic leak the system could be intermittent and, perhaps, go hand-in-hand with my earlier suggestion that the rapid pitch-up just before impact was normal hydraulic controls being restored, even momentarily, but still to late to change the aircraft's vector.