They look like Celmisia semicordata (syn. coriacea) or perhaps one of the many hybrids of this plant which have come from Ireland or Inshriach in Scotland in former years. Can't be quite sure though. If you've had it so long Ian, you are to be congratulated in a major way. At best, in lowland conditions, celmisias, especially the larger-leaved kinds, tend to be short-lived and do, in fact, rot off in the rosettes. Unless there are side rosettes, that's the end. You in the UK, especially the cooler parts, grow them more easily than we do, with hotter, drier summers. My personal theory about how Alan Furness is able to succeed so often, is to do with cold winds and mists from the north sea.
Perhaps yours has just come to the end of its natural life. Sorry to be so negative. Someone else will be able to give a more hopeful outcome.
