Depression is a tricky one. Don't quote me as I am not 100% however, I believe if there has been some key events that have caused said depression that are unable to happen again then there may be a good chance of appealing. However, if the depression is relatively undocumented and unexplained why it came on then I am under the impression that may be an issue.
Keep in mind though that with a role such as one in RAF Regiment they are going to have to be strict with such things.