Atos Complaint

Hi, my wife is going through the ESA IB transfer, she suffers with poly arthritus, fallen archers anxiety and depression and urinary incontinence.
She attended a (medical) on Monday which was horrendous, she went with her mum because I get so angry at these things I thought it best if I dropped her off and picked her up. I wish I had gone with her now.
When she came out she was so upset, she said the female assessor, or what ever she was who done the assessment was rude,got angry when my wife didn't seem to give her the answers she wanted, sneering at her when she said she needed help with toilet needs, asking her if she thought it was all right to have her husband cleaning her up after using the toilet,
then asking did she care for our son, when my wife asked what did she mean she said, well do you take him to school, my wife said no, she hasn't been able to do that for 3 or 4 years (our son is 13 by the way and has autism) she then went on to say, so you haven't cared for your son for 3 or 4 years then, asking her this 4 times till my wife was near to tears and her mum asked the assessor to stop saying that please she is a good mum and loves our son.
The assessor,asked about adaptions in the home, when my wife told her she can't grip anything so they would be a waste of time and our son is autistic so the change would cause our son too much distress, the assessor said well he would have to get on with it, then telling her that if she comes for a medical in a year or two the decision maker will not except incontinence in this day and age and she needs to see her gp and tablets for it and get pain management, stronger pain killers etc, because the decision maker will expect to see an improvement. I took her to see our gp this morning who agreed with me that i needed to make a complaint because this woman sounded like a bully,
I need to know who do I send it to and is there a standed letter?
Thanks in advance and I hope i havn't been to long winded in my post,

While you're best to wait till the report comes before making any complaint, you your wife & her mother should write down everything you can remember about the assessment and its aftermath while it's all still fresh in your minds. You'll want to refer to these contemporaneous notes later and they may well need to be produced at some stage and if I have it right may be regarded as evidence.