Okay it stopped raining...

Started her up (<1 minute, definitely getting the hang of it). Slowly warmed it up, turned off the glow, then gave it 20 seconds of full chat, then throttled back to 1/2 throttle (about 5000rpm). 1 minute 10 later it coughed and started banging away about to stop, so I immediately turned the glow back on and viola immediate sweet running again. I repeated this two or three times. So I agree its a problem with it richening and cooling the plug.

So I spent some time tinkering. Ooo, it's a sensitive little bugger. Going 1/4 and 1/2 turn richer on the regulator definitely aggravated the problem, reducing significantly the time before it would quit. Finally, I have ended up with the reg about 1/4 turn leaner than it was. At this setting, it seems to run consistently for >30s at a variety of throttle settings. There is a hint of hesitation when opening up and if left to idle for too long it starts to speed up a little, then it starts surging and eventually stops. So I'm guessing it's still 1/16 to 1/8 turn lean - it's gone dark and I've just run out of 20% nitro so it'll have to wait to the weekend for a bit more running and confidence building. If I go another 1/8 turn lean from where it is, transition is very poor and the needle is insensitive at full throttle, so I don't think there's much point going any leaner.

Do these behaviours and degrees of adjustment sound normal? If so can stop questioning whether there is still "something wrong" and get on with tuning some more.