Big thanks to the Access Norton forum here, where I discovered that my bike's been running like shit and kicking back so bad it's almost broken my leg, not because of the timing being out, but because of the bloody battery! My Commando isn't running points anymore and had a Boyer ignition fitted by the previous owner.
Anyway it turns out that the boyer will advance the spark if the battery is low, causing the violent kickback I was getting. I heard about this, and since my battery's real old now I thought it was worth getting a new one anyway, so have chucked in a new battery and she's running sweet again. And didn't kick back on me either! Which was a big relief. I was pretty tentative trying to kick it at first.
The battery I put in is one of these new fangled (actually I think they've been used in cars for a lot longer?) sealed gel ones. The guy at the bike shop up-sold me on it (I'm a sucker for the up-sell). he reckoned that it would hold it's charge a lot longer, especially in a bike that isn't being used all the time. I've always been a bit shit at remembering to top up my batteries too, so I thought this would be worth a go.
I'm wondering if I can chuck my charger on this if I need too? Or does being sealed mean you can't recharge them? Wish I'd asked him that?