on reflection, maybe all I did wrong 1st start was flood the carb because, of course, the automatic choke will have been full on. I should have been patient and waited a few mins before retrying.
On the second attempt about an hour later, when I had boosted my by then discharged battery, it did fire and stop, so I should have tried again with 10% throttle. But I think I just flooded it again.
It would be nice to hope it was nothing more complex than that...and that if I'd had a second battery, I would have been fine.
I am taking Neil Hubbard's advice and designing a new set of connections using EC3 connectors and a break-out to a couple of lugs for jump starting and charging, plus another plug in tamiya connector for trickle charging. Off to Maplins now.