Urgh. I hoped there wouldn't be problems with this character but i've been running into problems with the skin weights tool. Every time I paint weights, the program 'forgets' what I painted and reverts back to the old bad weights meaning that when you rotate an arm parts of the chest rotate with it too!
Well, we're working against the clock so as a solution i've had to use the (urgh) 'reduce polygons' tool to simplify the model and extend the arms out and use the rigid skin tool. It worked quite well, and if anything adds definition to the model as there are more bumps around the model which can only be seen with the outline- so it looks like he's mroe muscular. It's hard to describe but it looks fine.
If all goes well I might be able to smooth him again to add back the definition and make it more like the drawings of him.