I found time to have a look at it again at the weekend. The only thing I did was remove the impeller jug and blow really hard through it, both ways, to make it spin fast. It seemed a bit freer after that, so maybe there was a bit of grit in there or something. Because when I put it back in and tried the machine, it worked! At least if it goes again, I know it will only cost me £20 to replace that part.