By jimminy i think i identified and (mostly) fixed the problem. The bike now slows down instead of holding the revs! All i need to do now is put a bit of lubrication on the sliders again. They aren't totally dry, but not as lubricated as they could be as it is slowing down to idle over about 5-6 seconds instead of only a couple. I guess i should plain on taking the carb top caps off again, or just being lazy and taking the air filter out and spraying wd40 in the throat.
Next on the agenda is to balance the carbs and possibly re-tweak the idle as i suspect it's been out for some time now. Oncei've got that done the true test will be taking her out on the road and giving her a shakedown.