company doctor vs personal GP

[mandz2006]

i am currently signed off work by my GP with work related stress, i'm currently going throgh the companies greivence procedure due to being a victim of bullying, my employer has sent me to see the company doctor and thy have said i am fit for work but on light duties and i should return to work asap, over riding my doctors sick note surely they cant do that? Is it legal to work when your signed off sick? what if i have an accident at work will my insurance cover me?