Once the appeal is made it still has to go through the process. HMCTS can hear it in absence of any or all of the parties anyway, and can make any case management directions they think fit. I’d expect that the appeal would be heard but that it might be a paper hearing or an oral hearing with just an LA Presenting Officer, unless CAB are either directed to attend as the appellant’s rep or decide to do so. If there is an executor or family member they could take over to represent the appellant.
I would ask HMCTS for case management directions and see how they want to proceed. Part of that will include whether to accept it for hearing as a late appeal; my experience is that most are accepted as in the interests of justice, especially if there is a HB overpayment in issue.
