I now have more information.
Apart from Most Senior Boss (MBS), there is:
A - my own boss, who reports directly to MBS.
B - my immediate line manager.
C - a long-time colleague.
A and B have both previously had this role as part of their remit. Both declined the offer of picking it up again.
A couple of weeks ago I heard that C had been talked about in connection with the role. I have a good relationship with her, I rate her highly and think she would do the job well. When MBS approached me, I assumed C had declined to take it on, IE, I assumed he was working his way down to the bottom of the barrel, and that he didn't want to tell me that I was only his 4th choice for the role.
I mentioned these discussions to B, who was supportive but made it clear she thinks the role will be a thankless task. In her experience, MBS is impossible to please. I value her opinion and this does worry me.
I now have information from 2 different sources that C does want the role, and that she has the support of her own line manager, and mine (A). She will probably perform very well at interview. As part of her current role she's already put proposals to MBS about how she would provide support to him for an area that is under his scrutiny, but he wasn't very impressed. It's an area she has stronger experience than I do. Not encouraging for me!
MBS has not yet discussed his thoughts with A, and I have not had a chance to to discuss any of this with A either. A has always been a great support to me. According to MBS, A wants to keep me in his team (fine; but no promotion there). I will have to cite A and B as referees.
Aaargh. I can't help feeling that whether I apply or not, and whether I am successful or not, I will lose the support of at least one of the above - and possibly all of them.
Sorry for the long story!
I'm hoping that things will be clearer once the details are published, but any perspective on this is appreciated.