Well Maplin let me down - micro drill sets out of stock - so I thought again. Perhaps I could make a broach out of a pin or something? I raided Helen's sewing box and found a needle which the digital verineers told me was 0.69 mm in diameter. looking at it, I thought who am I kidding. I can't cut teeth on that, anyway it would have to be a pusher rather than puller and even without resorting to Euler's formula for the buckling of struts (see, I do remember something from that failed engineering degree - just the name you understand, not the formula), I could see that was a non starter.
Maybe I could use the needle as a lap - wrong way round I know, a lap is supposed to be softer than the material to be worked. But anyway I chucked the jet an mounted the needle in the tailstock, smearing it with polishing paste. With a spindle speed of 1360 I advanced the needle, occasionally withdrawing it to redistribute the past, and slowly, but slowly it began to disappear into the jet. Well something was happening, either I was opening out the jet or reducing the diameter of the needle! Once the parallel portion of the needle was well into the jet, I got the verineers out again and the diameter of the needle was unchanged - success!
Cleaned thoroughly and reassembled into carb - would the T300 now start ok and run off choke? YES! Put that in your pipe and smoke it Mr £82 carb man ;-)