Things have moved on somewhat since I posted above!
I bought a copy of the owners manual (£30) but it didn't really help me so I decided that I'd pull the stator and have it tested properly. Bought a flywheel puller (£10) and sent it off - turns out the ignition bit of it was screwed so had that rewound (£97). When I got it back it still wasn't having any of it but, after consulting the manual, found that the spark plug was wrong - so I replaced that (£2.50).
Obviously, cleaned the carb, checked the reed valves and all that stuff..
After that, it fired up - but there was a nasty tapping sound in the top end!
Took the head and the barrel off and found that it was badly worn, can't have been much compression but I didn't have a basis for comparison so assumed that's just how they are? Helps explain the starting problem anyway.
Anyway, the barrel is now away for relining (£150ish) and I'm shopping for a piston....
My cheap bike is becoming less of a bargain by the day, but I reckon I'm still winning (just)