... in case anyone else (like me) finds this in future...
There's a captive nut inside the black plastic gear cover, but it's not held firmly. So if you push on the blade retaining screw while removing or refitting it, the nut can fall out and end up suck inside the drive gear. It's relatively easy to fix - remove the screws holding the green side cover (one is under the label) and carefully lift the cover off, taking care with the oil tank cover. Then remove the three screws holding the black cover over the gears. Remove this carefully - there's a thrust washer on the drive spindle which would be easy to lose. Put the nut back in its socket (I used a bit of clear glue to hold it more firmly), then reassemble - black gear cover first, green side cover second. Finally, when refitting the blade, DO NOT press down on the screw to help it engage!