Magic Mouse sideways scrolling
I've looked this up a dozen times over the last year, but just after posting found a solution that evaded me previously. This does the trick. Its in the Accessibility settings tab, not where you'd expect a solution:

https://forums.appleinsider.com/discussion/106237/is-there-any-way-to-disable-magic-mouse-scrolling-in-photoshop-illustrator 

Its stopped the crazy scrolling.