Thanks for explaining things in more detail Evil. I've now implimented this code and it has fixed the border flicker, but now I seem to get another problem.
On occasions it's as if the VBL isn't occouring at the top of the screen as it should rather somehow it's delayed to the first actual drawn screen line thus it's knocking all the timer b interrupts out. I've checked this by setting a background colour on the first timer b I call and then set the whole palette on the vbl and sure enough the background colour stays changed until the first drawn screenline when the whole palette is set.
I don't have to make more blitter objects onscreen occour by firing to do this, I can just leave the game scrolling and all by itself this will happen at what seems random moments.
This is totally bizarre and is driving me totally up the wall that I'm thinking of totally stopping working on this game. :/