Hey Neostrider - thanks for taking a look. Actually I think I managed to sort this problem out. I am pretty sure that it was a problem with the normals - when I tilted the head the same thing happened. In the end, I used the set to face tool to make sure the normal was facing the correct way (this 'hardened' the geometry) and then used the soften normals tool to balance the areas out...if that makes sense.

I am still not sure what was going on though. It was as if the normals from certain areas were not rotating with the rest of the head. I am pretty sure it wasn't anything to do with painting weights as I spent quite a bit of time playing with that tool and it did not seem to have any effect on the dark patches. In fact the areas that were giving me problems were areas that I had tried to fix earlier using the edit vertex notrmal tool, so now I am wondering if it had something to do with deleting history or the way objects were grouped - it was almost as if certain areas were reverting to a previous state...anyway, is it time for beer?

Thanks again mate.