Thanks again chaps.
Changing the edges of the platforms has been suggested before, but it looks a bit naff in my opinion so I left it. I'm not going to change the colour register of the background as it will mean going through the graphics and changing them slightly, a bit too much hassle at this point. The best suggestion was to overlay a tall black sprite on the 'overscanned' area, however does anybody know if this is actually possible? I'd have thought it was unlikely as it's outside the area of the screen.
It isn't a great situation and it can be a bit irritiating - but it shouldn't prove a issue long-term (I hope) and in any case at least the AGA version will deal with this as I want to use parallax scrolling rather than a copper background.
As for highscore saving - I could do that I guess - a bit too used to WHDLoad now! At the same time it does keep disk access and disruption during the game to zero so it has its advantages.
(Oh and level 18 - hardcore!
(Oh and thanks DJCruicky for the different background suggestion and the kind offer regarding the icon, I appreciate it!)