Right - an update in the hope of some sage-like advice.
Firstly - the choke cable was absolutely knckered. New one on the way to me now (it was not closing the choke valve/brass thing completely). Should be here today, actually...
Second - I've put a free flow cone filter on her now, and can now get her started as long as I do two things:
1) The needle MUST be on the bottom of the five slots (i.e. it's richest). Won't start in any other position.
2) I turn her over for three-five seconds on 30-60 percent throttle, and then quickly back down to no throttle, keeping the starter button pressed and she roars into life and ticks over fine. (tickover is 1300 rpm)
Thirdly, she seems to rev fine all the way up to about 5500-6000 revs, then just loses power. Standard jet is 125. I've tried a 130, 135 and a 140 and it only seems to raise the max revs slightly. (Redline is 8000rpm)
Air/fuel screw is currently at 2 turns out - she starts anywhere between 1 turn and three turns out, but seems happiest ticking over at this. No physical way to adjust this whilst running, despite my ingenuity.
I'm hoping some carb sages are going to shed some light on this. Would the standard pilot jet be THAT effected by the changes, and how does it explain the bad starting before the filter change? I've cleaned out the carb, by the way, so I know there's no blockages going on here, and surely it would be a bugger to tick over if the pilot was dodgy?
*Bangs head against wall*
But I've learnt SO much about carbs in the past few weeks, which is a good thing...