Will she run?

So it started yesterday. Would it today? I wasnt planning on doing any more on this until maybe the weekend, but Hugh happened to be passing by and, well the temptation was too much.
We rolled it out onto the drive out side my house - and to my delight, she started perfectly.... every time. And with no more smoke than might be expected for a 20 year old 2-stroke.
I was just about to drive it off, when i noticed the real tyre was virtually flat - this was the first time id sat on the thing so hadn't noticed the tyre under any weight. Remind me to get a decent foot pump - mine is rubbish.
So we tried again. And this is where Ive got all confused. I can remember what happened, but it seems at odds with what in theory might happen :-/
Bottom line is - something in the clutch area appears to be stuck - I can put it in (and change) gear and the wheel runs freely, but i cant slip the clutch, so any attempt to try and drive it, simply results in a stall.
So it looks like a bit more tinkering to do -