If you dig the plant up, and the roots are covered with a WHITISH
network and smell mushroomy, then it has been killed by one of the
more common ones. No, I don't know what they are - and there are
probably hundreds that match that description. The solution is the
same in all cases (including for honey fungus):

Thanks for this. There was nothing odd about the roots, they were just
dead. If it means anything, the rose had always looked odd and had
always behaved strangely. tending to bolt, and the leaves never looked
right. I don't think it had reverted to root stock, because it looked
like the rose on the label.
John