Update 10th Jan 2020:
Added a small lead wrap around the stem of the schrader valve on the LH wheel to promote "drop to the bottom" behaviour and to demonstrate free running.
Here is the video:
So that shows the LH hub is fine and the brakes are not binding in any way.
Then the right was tried (same M.O.) and no dice. Quick to stop and random stopping points.
Here is the hub with caliper and with the same lead weight added to the schrader valve stem.
Quite a difference. I also tried it with the caliper removed and it still stops at random points around the rim. So there is a bind internally in the hub-bearings I think.
I took the caliper to bits and "correction", it
is single pot. So I removed the pads and the spring and jacked the piston out as far as I dared and washed it all with car brake cleaner and gave it a seeing-too with a fine wire brush.
I then played in-out games with the piston with the brake lever and a pair of pliers on the caliper/piston ...back & forth.
I have bent the pad-spring out slightly to increase its return-pressure and re-assembled the caliper on the bike.
I stopped then because it was getting very cold and a visitor had arrived.
Tomorrow I will set it up exactly the same as the L.H. Caliper and that will be as good as it gets I think.