| OK guys - Thanks for your words, and now for an update:
The battery is 5 days old so I don't immediately suspect it, but perhaps it's just not a good un'.
The flash def came from the neg terminal (my face was only about 12 inches away from it at the time and I nearly pooped myself) - and it also left a grey sooty deposit around the terminal.
I've cleaned up both terminals with a bit of 320 grade and tried re-starting it again....
Success!!! She fired up as normal and I took a reading of 14.4v across the terminals @ 3000rpm so she appears to be charging fine.
As the only thing I did was clean the battery terminals it must have been that - but as I said before, it's a spanking new battery (unless, do Yuasa coat their terminals with something to protect them during storage? The last battery I bought was a Varta). | 
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