Well. I let her alone for a bit as I was getting stressed about it. eventually took all the jets out cleaned them all and put the top back on. Nothing. Turned her over while craning over the bonnet to watch. No fuel flow. I had recently changed the fuel filter and noticed at the time that all the top fuel hoses were new, while the one the tank side of the filter was old. When I looked today, the end of the filter was poking out of the side of the hose. Hmmm. carb cleaner overspray melted hose maybe? Either way. SOLVED.

WOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!

ah, so you had a fuel leak causing underfuelling? Glad you got it sorted dude, it's always nice to get things fixed, and even better when you know exactly what the fault was.

Glad you found it before it found you.

Did it not smell of petrol then?

Rarely yes. But I just thought that was a symptom of the carb probs, besides the split was before the filter and rather than spurting petrol out, it wouldn't suck it up. Lucky really, could have been nasty other way round.