Ok some progress...
Thanks to the discovery by Tokra
that position 68 gives identical behaviour on the real h/w and VICE - I have modified my code to achieve a stable result on both using this value. I've also eliminated the artefacts present above the lettering in ex4 (real h/w) by setting that line of the color RAM to the same colour as the background within the 1st raster split and then restoring it to the colour needed for the scrolling region in the VBI
I now just need to work out how to get more lines of text without flickering, although two would provide a bare minimum of information for some games if this proves impossible.
See attached ex6..(btw there's no screen jitter during scrolling on real h/w for those that only have VICE, just a small hiccup on direction change due to a bug/defficiency in my code)
btw, you can also increase/decrease/reset the black bar raster timing delay on this version using the keyboard.