That KMC chain is not directional.
If that's your bike in the vid, the top jockey wheel is a long way below the sprocket it feeds the chain to.
It should be closer and further forward (in that gear it should be well forward of the axle of the wheel and the chain between sprocket and jockey wheel closer to horizontal).
Check B screw adjustment in large sprocket.
Thats very interesting, I just went out to the shed and had a play with the B screw, if I went too far it didnt shift nicely into the 36t, so I backed it out until it still sifted up nicely, its now much further forward and closer through the rest of the casette.
This made it slightly quieter, but it still sounded noisy.
So whilst I was there, I pulled the cage off and took the jockey wheels out, I had spares to put in, so thought why not, and I popped the rubber seal off. Needless to say it was dryer than a monk on sunday! So I packed them with grease and put the seals back and re-assembled it all.
It's now ~70% quieter. So I think this was the root cause. Obviously the drive train makes some noise, and the chain is probably a bit worn, but I'm now happy with it.
Thanks for all for the suggestions!