Thanks AS
I think you may be right with the mid grey if only for continuitys sake. The other thing I noticed was when loading the tex you can't just copy and paste to the bump channel you have to go via the alpha - or am I missing something.

On a workflow note
I sprayed these texture directly on the model in (the much underated) metasequoia cause that seems to me easier than trying to work out which poly is where in PSP for example from the exported  UV map. Any tips?