I'm not going to argue the point because we are approaching it from different angles and have differing experience so in essence we are both right, BUT the point of this question was that Scarlett felt that her last interview had been fabricated- therefore we are here to help Scarlett, not ATOS, therefore because recording them is NOT illegal and cannot possibly do any harm it's worth doing. If at a later date there is a difference of opinion she has considerable leverage against them- she can produce it at a tribunal who may or, in your experience, may not accept it, if they don't she can threaten to flog it to The Sun and make a big hoohaa, in either event it gives her more of a chance against them not less of a chance, which is, I assumed, what we were trying to facilitate.
So once again- it's not illegal but they may or may not accept it at a tribunal- in either event you do not need to ask their permission first if you do not wish to, but if you do and they agree it might make them more careful about the way they handle your case in the first place.