Is this an example of what backlash can do?

[Jonathan]

Note my cutter seems to get very close to the sphere on the roughing toolpath, this was something I didn't expect and a potential problem, I presume this is solved using longer cutters?

I think you're referring to the chuck colliding with the sphere? If so then yes a longer cutter is the way to go. Be careful though - cutter rigidity is inversely proportional to the cube of the length of the cutter.