Manually deleting the .ptc and .bkm in /rmantex seemed to help.
I also found, if it's useful to anyone else:
My renderman resolution needs to be set really high, makes sense seeing as it's creating a texture so this determines the res of the baked map.
Should these be kept to square values ie 512x512, 1024x1024 etc..?
So that's a mystery solved but....
Works fine when I have one sphere on a plane. I then add more spheres and rebake, now all the new spheres, and the old one now have weird blocks on their texture it doesn't look right. I assumed this is because there's more object so the bake needs more resolution, so I set:
Renderman RenderGlobals: Display Resolution: 2048 2048
And rebaked. No joy. Pretty much exactly the same problem. Delete these new objects. Rebake, and it looks good again?!?!
Seems like I can't make any adjustments to my scene even if I rebake which just seems wrong to me. Where am I messing up?