Just an update on this.
Spoke to my colleagues about my thoughts and got some constructive feedback.
Going to start with HR to find out if there is a metric being used to decide your grading -> manager -> dept head.
Is your colleague your manager? If not then you are wasting your time as their experience with this is limited to their experience on the matter.
Take you work contract and responsibilities, highlight the things that you do and add the things that are not in the current contract. Take that to the manager and have "the talk".