I have a 3.2mm motor shaft and a 9 inch prop (sorry about mixed measures!) and I have a 4 piece adaptor comprising a nut, washer the split bolt thing and its housing. The Prop is plastic and the hub has a textured side and the other side has "Made in Germany" printed on it in raised lettering. This lettering is a main in the butt because it causes the prop to go on wonky. What is the correct way to mount a prop on a motor?

Currently the assembly is like this...

Motor <-- split bolt <-- bolt housing <-- prop <-- washer <-- nut

You have it right. As you tighten the nut the split shaft compresses and centers on the motor shaft. Hopefully the prop will streighten out as you tighten the nut.