Hey, thank you for coming back so quickly. Hugely appreciated.
Ok that was REALLY helpful. Using that technique, I realised that I was falling through my subroutines rather than jumping strait to Overscan once I had reached the 192 lines. This meant that I was adding a few additional scan lines. Thanks for pointing me in the right direction and thanks for the additional hint on the way I was attempting to address the play field pattern values. That has cleared up something else that was confusing me.
Now I have it stable I would like to understand how to make it scroll again in a controlled way.
If you could help me understand how to approach the scrolling I would really appreciate it.