Jackal, FURAR -- Bloody genius! It only worked!
Well, sort of. Undoing the screw released a few ml of water and fuel. But then when I turned on the fuel valve, there was a massive leak, seeing as how it turned out not actually to be a drain screw, but the bolt holding the whole float bowl in place. (There was way more coming out than would have fit down the threads .....) After turning off the valve, undoing everything again and refitting it all properly, I turned it back on and there was no more leak. Gave the starter a pull, and it sounded like it was going a bit. A few more pulls, and it caught and stayed running for about a minute. Then it died. A few more pulls -- sounds like it will catch on the next one -- flooded it. Got it going again, a bit longer this time. Choke off, it kept on running ..... Then it died. Out of fuel.
But, it runs, is the main thing. Not as sweetly as it should, but it's definitely sortable. Further work is going to have to wait till I have got some more petrol and 2-stroke oil. Running fresh fuel through the carburettor will help, yes? I suppose in the meantime I can at least clean and re-gap the spark plug .....