Please forget about my last query - i think this was happening because I had imported the linework as an image but not onto its own layer.
I am now having a slightly different problem - when I import an image file containing some linework I want to use I now import onto new layer. I am setting blending mode to multiply so the white areas are ignored and the paint beneath will show through.....unfortunately though when the image comes in it seems to be duplicating the canvas texture so that the canvas appears darker than it did previously. If I use the eraser on the new linework's layer it erases this darker canvas area to reveal the original canvas below, but obviously it wouldn't be practical/desirable to rub out all the white areas between the linework!
The attached PDF should show what I mean - if you look to the edge of the linework you can see how the canvas is lighter outside of the linework's layer. Basically the areas I want to leave white are going to be darker because of the darker canvas texture.
Sorry for the lengthy/less than clear explanation, but hopefully there is a simple solution!