Finally I content aware from the UV edges of the map, this ensures if the texture is scaled down (128, 256, 512 etc) there will not be a visible UV edge line from the lower res blurrier texture.
The normal map for this model was created directly in ZBrush, subdividing the mesh a couple of times allowed me to get more detail in to the model and applying a surface noise effect to the entire model helped bring out some finer details.
This is an older version of the model, I refined and changed the higher-poly Zbrush mesh afterwards but it shows an example of the process that takes place to produce the normal map.