After a couple of hours I think I've finally found the settings for my NN5 with EOS 5D Mk II plus EF15 (fisheye).
Six shots round (60 degree intervals) in portrait orientation with the camera tilted down 15 degrees.
Lower rail set at 57mm and upper rail at fractionally under 80mm.
The attached preview from PTGui is what I'm looking for if I understand things correctly - a perfect circle with no "saw teeth".
I have yet to tackle the nadir adaptor, but there has been some good advice about it here recently - so I will work my way through that. I assume that using the nadir adaptor will not affect any of my settings?
Next task if everything's fine is to try a zenith shot and then have nadir fun!