Thanks everyone for you replies. I contacted the CAB today and they agreed with all the advice given here. Letters of appeal have been drafted and will be sent registered mail along with all previous letters sent to ATOS.

My Son suffers with severe mental health issues, and originally applied for ESA in 2009 During times when he is ill he moves in with me as my ex wife cannot cope. She has had mental health issues for 30 years. My SON applied for ESA and had his original ESA medical in January 2010 after the assessment phase, he passed this and was placed in the support group. In September he was asked to complete a second ESA50. My son is listed a living with my ex wife, so all DWP correspondence is sent to her address. Unfortuantly due to fact she was hospitalised on and off for 5 months this year a lot of correspondence from Atos and dwp WAS unopened late. This was all explained to ATOS In a letter , They then subsequently requested a new ESA medical assessemnt and gave a date to attend. I phoned outlining the fact my son's condition had worsened and that I could take no more time of work because i took a lot of time of aiding my ex-wife. My son is unable to attend the medical alone, he does not leave the the house when he is ill. I requested a new appointment with a weekend date or a home visit, they said they would pass the message on to the relevant department and they would contact me in due course. No one called me back or wrote. They then sent another letter with a new date for the medical which again was a week day , I phoned again outlining the points made in my previous call, I then wrote to ATOS, detailing my son's health issues and the reasons why we required a weekend appointment or home visit. The next letter from the dwp was in January which was my sons p45 and various letter stating he failled to provide reasons for the missed medical. They have deemed him fit for work they day after the medical date. DWP say they wrote to my son asking for reasons why he did not attend the medical but, i never saw this letter and I open all his mail that my ex wife passes on to me. My sons health is now probably worse than when he had his original assessment which he passed. Please can anyone help