The story is - the vacum/boost hose to the waistgate from the turbo blew off while the car was on the dyno running at full chat, which caused the waistgate to shut so all the exhaust gas had to pass through the turbo, this then caused the piston to melt and also vaporise the outer fins of the turbo turbine wheel!
This happened the day before the daddy thrash. However the car has still covered over 1000 miles since.
I never noticed the reduction in performance because I have been using reduced boost to prevent the Felpro head gasket from failing.
S.