Well, this has proved fun! For some unknown reason, the entire circuit decided to stop working regardless of which transistors I installed! I ended up having to completely start again.
Eventually, I managed to get it working with the BC327/BC337 transistors at 3V supply, only to find that the maximum unregulated voltage was barely 300V. Even allowing for a little loading by my 10:1 HV probe, this is too low.
So I played about with the transformers, with interesting results! It would seem that the audio transformers I have are not 1:1 isolating transformers, but output impedance matching units - put them in the wrong way and the current goes up dramatically!
In the end, I found that the unidentified transformers taken from the old emergency lighting switch-mode inverter worked best. With one of those fitted, i'm measuring just over 400V unregulated. How much higher the true figure is, i'll have to wait for the 1GΩ resistor to find out! I also found I had to lower the oscillator capacitor value as the 100uF was causing visible pulsing! 4u7 seems a good value at present.
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