No positive cause was ever identified. The a/c simply departed from controlled flight. From an altitude (I think) of around 800', the aircraft gently entered a vertical dive. The only transmission made was thought to be the middle portion of £@cking he!!
Could have been a manufacturing fault. Could have been hydraulic contamination, could have been undocumented major engineering.
Exceptional guys, every single one.
Just fortunate there were witnesses, otherwise we could be looking at another case of 'Gross Negligence', as in the tragedy on the Mull of Kintyre.
Never forgotten.