Gone and given it a bit of an inspection this morning and found the ground electrode on the l/h plug is much darker than the r/h. I couldn't smell fuel in the box oil, but since the problem has only just occured there's no real surprise there. Gave it a couple of kicks and it started up as easy as usual but the l/h side is definately smoking way more than normal, while the r/h one is just fine.
The engine (told the top-end was done 8000~km ago) had been sitting for a few years as I picked up the whole bike in bits after the last owner lost interest before he started his planned rebuild. I've been riding it to work every day for the last month or so with very few probs, and if anything the engine seemed to be feeling stronger the more it was riden, so this was a real shock and a great disappointment.
I'll probably take it for a bit of a run later to see if there are any other symptoms, but I can't think of much else it could be unless the l/h carb has suddenly started to run ridiculously rich for some reason.
Any other thoughts?
Got it started and let it warm up a little while I got my helmet and gloves on as I usually do, but when I climbed on I forgot to give it a little blip of the throttle as I put it inot gear and the thing stalled. 
Who would think that could be a good thing? 
It was only when I looked down to flick the kick-start out again that I noticed the little the damp patch on the concrete under the vent hoses. A quick inspection revealed it was fuel which I'm pretty sure is comming from the l/h carb overflow pipe. So hopefully it's just some crap under the float needle thats over-fueled that cylender causeing the smoke. [-o< [-o< [-o<