My wife and I have had plenty of experience at the hands of atos of the last few years and to anyone going through assessment I would offer this advice. The assessment isn't to see if you are I'll, it is to see if you get benefit (under the guise of fit for work). The assessment is points based and it is vital that you understand this system. You then need to understand your desired outcome and the points needed to achieve it. Look at the sections of there assessment which are applicable to your condition, understand the descriptors and what they score and ensure you have the medical evidence to support them. Supply all the evidence with your forms. You will then go to an assessment. When asked about the descriptors that you have chosen stick to your comments in the form and ALWAYS answer based on your worst days. You them are at the mercy of having a decent assessor or what their monthly targets are ( the latter being a personal opinion based on the variability we have seen). If they deem you fit for work, appeal. The appeal is based on your condition at the time of the initial assessment. No change in your condition since then will be considered. They will ask you if things have changed and you must say you are the same or the appeal is null and void and you will have to apply again. No further evidence since the original assessment is likely to be considered so it is vital that you have everything in order from the start.
Do not count on being treated fairly as it is unlikely you will be. You need to play the game which is a sad state of affairs.