As long as you have been going to your doctor's or seeing your specialists on a regular basis, you should not have a problem with the visiting Atos Doctor, but this is still not a guarantee that you will be shafted or misunderstood by the visiting Doctor.
Just make sure you have someone sat in their with you, your partner or a friend.
If they ask you to do something "eg" you have a bad back and they ask you to walk up and down stairs, and you know this will give you even more pain tell them you are not prepared to do this and stick to your guns, also watch what you say to some of their questions because the questions are designed to catch you out.
Remember you can give a written statement of how you feel and your conditions which are affecting you to the visiting Doctor also make sure you have any medication that you are taking for your condition at hand or an up to date prescription, a lot of the visiting doctors who work for Atos are not very good with the English language (you can work this one out yourself) if you don't understand them, asked them to repeat what they mean, if you are still having a problem with this asked them to leave, and then ring up Atos and ask them for a doctor who speaks clear English.
Remember, the ATOS Doctor isn't coming to question what is wrong with you, the're coming to measure your level of disability and to see how badly you everyday life is affected by it. So act accordingly....