ESA assessment date

I have today received an appointment time to attend an assessment centre to speak to an healthcare provider about my ESA claim. I Completed a form as requested in sept 2015 and received a letter in October 2016 saying my benefit is to continue so I'm shocked to receive this.
I have been on IB since 2003 and then migrated in ESA in 2013 in the support group.i have Crohn's disease and rheumatoid arthritis. I have lost the majority of my small bowel and suffer from bowel incontience? I have to have immediate access to toilet facilities otherwise I soil myself. I rang the help desk as I had put on my form that a home visit would be preferred due to this and explained the situation and was told tough I have no option unless I can provide evidence via fax that I soil myself. He redirected to the assessment centre who was a lot more sympathetic and said she was going to look at my file put couldn't promise anything.
What can I expect from the assessment.
Any help greatly appreciated