Yes, it's nicely sealed all around now, it wasn't before. I couldn't resist and have just refitted the carb and given it a try. It doesn't seem to have made a whole lot of difference, but as it's after 11:00pm here I can't say I tried all that hard. Sleep on it and same again tomorrow I think...

Thanks for the help!

Mentioning the OKO (where did you get it, by the way)

Just bought it from one of the ebay sellers, can't remember which one as I've bought a few over the last couple of years, some based in UK, some Taiwan. Never had a problem with delivery from either.

Small change, but still no start. Screwing the air screw in or out incrementally seems to make very little difference. Hmmmm. Back to the drawing board.