Clutching at straws here, but could it be a dodgy fast idle/cold start mechanism? Might be making it run over rich which means it would start but then proceed to choke itself? Or not giving a sufficiently high enough idle?

Have you tried starting it with a handful of throttle?

I know you said the last tank was the "best fuel it's had", but despite it being 95RON of a reputable brand, it still could be full of sh1t!

My first port of call would still be to ditch what's in the tank and try some fresh stuff.

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