Mr. Alistair Carson, did you resolve your rear cassette/freewheel issue? Like Ann and JR suggested, you either have a) threaded freewheel OR a splined cassette. This would be around the axle on the opposite side of the disc brake. Its either threaded or splined, period. If neither condition exists, the next most likely scenario is they sent you a front drive motor kit instead of a rear drive kit. The front drive kit will measure approx. 100mm between the flanges which mount between the fork tubes, The rear motor will measure approximately 135mm between the flanges which mount in the rear dropouts. They are NOT interchangeable. The rear motor will not fit between the front fork dropouts, and the front motor will drop inside the rear frame with no axle extending out to bolt on. On certain hub motors, the mounting system(threaded or splined) is contained in the side motor cover. On my ebike hub , you can remove the side cover and relace it with a new mounting cover plate. Sometimes the cover plate cracks do to heavy stress in the highest gears (smallest cogs farthest to the outside of the bike). Replacing the side cover is much less expensive then replacing the motor. If for some reason your motor was manufactured without the correct mounting option, this could be one solution. I'm assuming its a brand new kit, hopefully the vendor will swap it out and get you rolling shortly. I hope this helps identify your issue.
