ELEKTOR FORUMS

on the face of it , appears a poor design. any component failure can pump 400V DC to the regulator chip
perhaps one solution could be , to adda series capacitor of say .01.1000V in series with the fuse F1
this would help increase the impedance and the damage could be minimized.
before switching ON, better to check the Transistor and FET for a possible faailure.

I am also of the opinion, that the circuit functions only when some sort of load is there.
thus you may have 1K terminating resistor across the 5V output. ( if at all regulator works)

HEAVY LIFE RISK IS INVOLVED AND PEOPLE DOING THIS CIRCUIT SHOULD UNDERSTAND IT IS LETHAL TO LIFE.