esa assessment period

I've been on esa since 6th June 2016 and I had to phone to get the forms for the medical which my support workers helped me fill in and I returned for the 19th January 2017 how long do I normally have to wait for a medical as I need to arrange support to attend with me. also is it correct that if I'm placed in a support group I would be entitled to a back payment of the difference of rates to my 14th week of claiming . I've already checked with Turn2us and they have checked my benefits are correct . thanks in advance for any answers I might receive