Further update after disassembling the motor. Generally the motor is clean inside, no sign of water ingress, with plenty of grease but I would still recommend cleaning and repacking at this mileage. CYC have used a very nice waxy white grease which is brilliant - because it shows any wear and metal contamination very clearly. I would consider there to be normal amounts of metal contamination on the gears at this stage.
However there seems to be 2 problems, one the has existed since manufacture, and the other during use.
1: The crank bearing in the centre of the chainring retaining ring is not concentric with the axis through that side of the motor. This introduces variable drag as shown in the previous videos. I don't have a dial gauge to measure this, but it is bad enough that I could probably measure it with digital calipers. :-(
See this video:
2: The large bearing that mounts the output gear is stuffed. This is a 6816-2RS bearing, and shows signs of water damage. This is a bearing that is very exposed to the environment and in a particularly dirty location. The rubber shields are not enough to protect the bearing - there is a large retaining ring (use a pin spanner to remove, is reverse threaded) that protects the outer edge of the seal, but the inner edge of the seal on the outside of the bearing is located under the clearance gap between the retaining ring and the chainring and there is no extra protection. Even a thin plastic shield may have made a difference, but surely there is a way to fit an o-ring or proper seal in there?
I cleaned out and flushed the bearing and repacked it, and it will run a little while longer, but it is irreparably damaged and needs replacement. The old bearing needs to be pressed off the gear, has anybody done this? How tight is it?
Video: