Pip 7110 change in law

My husband was on DLA in 2015, when they moved him to pip. He was on high mobility and on moving they dropped his mobility to nothing so we ended up at the tribunal where they gave him standard rate mobility. This April we got a 7110 letter saying there had been a change in the law about psychological distress while making a journey. I rang them and they said as he'd been to tribunal we had to apply for an appeal to the upper case tribunal, which we did and was refused. I think due to being out if time from 2016. We couldn't find any legal advice until a fluck finding last week. We have been told we need to find someone who I'd will to take a class action on. My question is has anyone else had this problem and does anyone know of any solicitors taking this on? please