I decided to look at the fuel aspect of this since I wasn't getting anywhere with electrics. Before checking fuel pressure, wanted to see if fuel was getting into the carbs when accel pressed. Took off the zip-tied filters and tried one - it sprayed. Silly me didn't check all of them, but will have to do that next time. I put some carby cleaner down them and had another go at starting - worked! Voltage at battery when running was 14V
OK I don't have a timing light yet, but thought I'd fiddle with the timing anyway and see what we could get.
I turned the cap CW to advance it some more and the idle increased (is this normal?). Seemed to run smoother so took it for a drive. No good - possibly worse than before. So I went the other way. It was a bit rough on idle (and even had a backfire then died on me) but driving results are GOOD. Not great, but good. I have no idea what the timing is of course but it's as far back as the adjustment will let me go.
Here's a vid. Sorry it's not very exciting. But you can see how the idle gradually decreases and backfires at 0:42
Is this fixed? DUnno, will see how it starts again when cold. Once started I'm happy to take it to someone who can properly tune it to run at its best.