Many thanks for all the advice, guys - given me a few things to look at, but it might be a few days before I do. I'll report back in due course.
As for the rings, I don't think these are necessarily a problem - I almost certainly overfilled it a bit with oil so it's probably blowing out the excess. The plug isn't oiling at all, when the engine dies the plug is black but still dry.
RT - the engine dies after just a few minutes not half an hour - sorry if I gave the wrong impression - so it could be the coil. Though I did check the spark after it died last time & there was one, though I might not have checked it immediately, or it may be that the spark isn't strong enough to fire under compression - I'll check again next time.
I'm not sure if it's the right plug or gap - the guy I bought it off said it didn't have a spark plug so he took one off a motorbike. As we don't have a Clinton manual, would anybody like to hazard a guess as to an appropriate plug and gap?
Points about exhaust & belt guide duly noted. Also re: air filter - I'll get a filter element before I try to set up the carb properly. Smud6ie - yes, I see the strings of numbers you mention. I'll check the things you & Vit mention & set it up accordingly.
RT - this is the bit that's confusing me - the governor vane. What is it exactly you're referring to? The throttle cable operates a sliding bar with a spring connected to the throttle lever on top of the carb (see my last pic). However, there is another (return?) spring (not fitted in the photo) that attaches to the rear left of this lever that also connects to - what? I haven't managed to figure out what it should attach to, consequently the throttle will not close completely as it's still tensioned by the other spring, unless I manually close it. So, is there something missing?