It improves somewhat with slower feed, the flute is longer than the material is deep. I just found one source of the problem the bolts holding the Y axis plate that ties the Z to the Y ballnut were not cranked down, they had worked themselves to just loose, nust have forgotton to loctite those one...

Anyway this tidied up the top diameters which are now spot on again, but with the 4mm bit the hole bottom is still smaller than the top, error is much less, maybe 0.05mm. This is enough to be annoying as you get a decent H8 fit to the shaft on one side and but tight on the other, opening up the hole doesn't solve the problem as it becomes loose on the top. Doesn't matter at all for normal profile work just shaft and bearing fits. I will stick with the 6mm known quality cutters for doing these.

I also just ordered a hopefully decent set of collets from Arc, not convinced by the much cheaper set I have based on the damage they take when you break an endmill.