Thanks for all the comments and feedback folks, I appreciate them.
Just to ensure that I hadn't recorded a random clump of grains of light masquerading as Nereid, I stepped outside again 2 nights later and took another series of images with the same gear and Nereid appeared where it was predicted to be.
I'm glad that this far off satellite of Neptune wasn't motoring along any faster, as its photons may then not have accumulated enough to record its trail.
Here is an inverted image composite showing the movement of Neptune, Triton and Nereid between 1st and 3rd Sept.
I have done a copy/paste of Neptune/Triton from a single frame from the middle of each series, to overlay the smeared trails due to their real-time movement across the frames in the final aligned/stacked images.
Cheers
Dennis