Cracked it, the 0.22uF (electrolytic sort) allied to its 30v zener was to blame, though acting as a "capacitor" in checks turned itself into a resistor when voltage more than my meter (to see charge up action) was applied, changed it for a normal poly type. Also changed 2 x 100K feeding it plus a 16K with 1% metal film now output is 230v with a respectable wave form.
I have changed other "flaky" (technical term) looking capacitors with good brand low ESR types, going to improve power rail tracks with wire support bridges as this copper track is lousy thin. Thinking of lowering its 8 x 30 Amp fuses down to 20 Amp which at 12v gives a total 160A with 1,920W (if my calculations are correct) so a cushion of 400W above its 1500W rated output, at least they may blow next time rather than cleaning up its FET,s legs splattered all over.
Can anyone suggest any other tips to improve its durability please?