I can understand where you're coming from - I've been signed off work for nearly seven months now. Unfortunately, in my case, I have found that Occupational Health can override the decision of your GP - my GP has been saying since my first week off that I should be at work but due to liability my work have taken advice from OH rather than my GP. I was advised by my HR that the doctor from OH overrides a GP but not a specialist [a psychiatrist etc].
My best advice would be to seek advice from a company called ACAS - if you search for them on google they've got a pretty in depth website. They have really helped me with my fight to return to work and were really nice to talk to. Also, there is a leaflet on the Governments website;
www.dwp.gov.uk/fitnote which says all about the doctors fit note and what employers should be doing about it. It might be of some use.
I hope things get sorted for you without too much stress.
x Katie x