This comes as a massive relief. My last official communication with $OLD_BOSS, where he denied that the role was redundant, made me feel like the problem must therefore be me. I'm so relieved $DEPARTMENT_BOSS sees things the way I do and confirmed my original thoughts.
Of course, this does mean that I need to find a new position within 2 weeks or be made redundant, the precise remuneration details of which I don't yet know. But at the moment this is a small concern as I have savings that should last me at least 6 months, should it come to that, and I have a chance to change my focus.