That washer with a hex hole in it is for the spur gear, I checked the manual. Plus, the crankshaft doesn't have that kind of shape. I've managed to assemble the clutch so that the clutch bell rotates freely, but the 4 or 5mm wide plastic spacer that comes with the thing is too wide to let you tighten the screw on the end of the crank shaft all the way.

Tighten the locknut on the slipper clutch all the way; as in until you simply cannot move the locknut any more while holding the spur gear? What are the dangers of having a slipper clutch that won't slip?

I doubt I have my diff on the wrong way round, considering that both diff cases come pre-assembled. If I spin the spur gear anti-clockwise, the front wheels rotate forwards, the back wheels do nothing. If I rotate both back wheels forwards, the rear shaft leading into the gearbox (and the disk brakes) rotates as it should, it just doesn't seem that any gears within the gearbox rotate when this happens (looking through those 3 small windows). It looks like the only thing that may be wrong here is with the gearbox, unless the two back wheels engage somehow when the motor's running. I don't know.

Any other ideas? I'll be taking it into the LHS either tomorrow or on Wednesday to see if they know what's up unless you guys can tell me.