Ok I figured it out.

In case anyone has a similar problem, I’ll explain what was happening to my model.

I had started painting weights for the shoulder joint, and testing it after painting, but because I hadn’t yet weighted the elbow/wrist/hand joints, they had incorrect waiting which of course was affecting the mesh whenever the shoulder was moved as they were part of the same movement.

So I checked through the weights for each joint before testing and it’s now ok!

Thanks