I've had a play and performed some minor tweaks. Here is the result:

ZhentMap.png

By turning the coloured strokes to a burn layer and giving them a texture they've been broken up a little and don't stand out quite as much. The outline of the sea was harder. In the end I used a layer mask to shrink down the line width and this also broke the lines up a little. The variation in the line width helps I think and I was able to give it a light texture and also changed it to burn.

I'm not quite satisfied with it, but I think I would have to redraw them with my tablet and pressure sensitivity to get them to blend into the hand drawn elements properly (tablet is at home right now and this map has had quite enough done to it!).

On a separate question. I know that photoshop people can shrink their selections by a given number of pixels. Does anyone know how to do this in Gimp? Thanks in advance.