You may be asked in to see your manager to arrange this. This is where you tell them that you would rather take the advice of your GP/counsellor/psychiatrist rather than take the advice of a doctor who doesn't specialise in mental health issues.
I don't know if you're seeing someone with regards to these problems or just taking prescribed medication. But it always looks better if you are seeing a counsellor or psychiatrist or psychologist.
If you wish to PM me I will try and help all I can with this matter
Firstly, when you go to jobcentre, take your current sick line with you,and if possible ,supporting statements from your coucillor! As to the medical, sometimes a face to face works in your favour, and hopefully this will be the case here.Again, if your own Doctor is prepared to keep singing you off, so much the better,this is obviously a difficult one, as in real terms, (And be honest here) do you feel you could do work as long as it isn't with Royal Mail? As you are having a face to face, it will be that decision you will have to appeal against, rather than the remote one via phone--so hang fire on that one. As I said in a previous post, if Doc keeps signing you off, then you could be kept on payroll (Even if not being paid) for up to two years. As to forcing you back to work,obviously, if your own Doc eventually feels you can return, and ATOS feel the same, then you would have to consider going back, or lose your job, but hopefully, this will not happen, either way, good luck.