Watching the github repository for changes to the fan driver, I noticed a PR was created to implement the missing hysteresis. I've been periodically updating my RPi to see how soon it would make it's way into a release branch and today updating from 4.14.71 to 4.14.78 has added this missing feature.
My Fan now turns on at 50 DegC and off at 45 DegC. No more rapid spinning up and down for the little fan as it hovered around 45 DegC.
The updated kernel isn't yet being pushed out via the normal packaging system, but is available if you run the command:
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sudo rpi-update
Martin