It's not a reservoir, it's an header (expansion) tank. As the coolant heats up and the pressure increases (pressure increase is needed to raise the boiling point of the coolant) there needs to be room for the coolant to expand. That's why you never fill the header to the top or you end up with coolant running out the over flow pipe until it finds it's level.
I seen that you were on that Kawasaki forum as well Stu, it's full of headcases. I had some guy telling me that I was doing the valve clearances wrong on the ZXR and that it doesn't have shims. I stopped posting on there after that.
Other ZXR "experts" also on there saying that the way the manual illustrates to carry out the valve clearance checks are wrong. Unreal.