A new update again, thanks for the suggestion, ive still learning.
I always have suspected my linear rails as a problem, so i did something naughty.
Ive honed/de-burred my rails(using knife sharping stone), to make it more smooth and having less preload.
I have to redo it, as at Z height 20 it isnt great yet, but half way the Z is much nicer:
Im getting there!