From an old sky box someone donated - only the PSU primary was traced as I
am interested in how they do MOSFET versions of the blocking oscillator
style self oscillating chopper.

The type of chopper is very handy if you want about 350 - 400V from 4.8 or
12V by reversing the function of the primary & one of the secondary
windings.

I've done this very successfully with the old bipolar choppers, but medium
voltage/ high current transistors are not always easy to find - MOSFETs with
these attributes are old news, there is however continual advancement in
ever lower RDSon.