I think maybe I didn't explain myself too well.
At this stage I am able to use all of the chip's RAM with a pure PBP program.

I think the reason why my code originaly stuffed up when I used more than a
certain amount of RAM is that I had asm routines that accessed RAM without
any bank switching.
Therefore, the addition of more RAM to my program might have allocated
existing variables to areas of RAM that would require bank switching by my
asm code when addressing them.

I hope that makes better sense.. everything is working now I have gone back
to pure basic. At some stage I like to convert it to all asm for optimising, but
best wait until it's close to finished.
Art.