Hallo Terry again,

I have built-up the circuit and made some measurements. The only change was the use of BC550C instead of BC550B, because box with BC550B was empty... I took this into account by doubling base resistor.

On oscilloscope I could see, that voltage at gate of BS170 makes a linear rise-up to 0.49V on negative half wave and is ZERO at positive half wave. 'Zero' means: Less than 5mV!
Transistor begins to discharge capacitor, when transformer voltage is about 1V. It needs 20µsec for totally discharging 100nF capacitor down to about 10mV. It then takes additional 50µsec for discharging capacitor even further, down to zero (<5mV).

So, circuit behaves exactly as calculated. There's no need for making a change. But, of course, circuit should be tested after fabricating, whether voltage at capacitor really falls down to zero, and really does not rise-up to more than 0.5V.

Bye,
Kai