My impression of the Mark Latimer character from S1 is a weak-willed, irresponsible father who showed little to no emotion from his personal tragedy. Mark Latimer at times seemed dodgy and annoyed at the proceedings, and generally more interested in protecting his own personal interests than pursuing justice for his son. Maybe that was a bit of Andrew Buchan or a bit of how the character was written into the script.
But by the beginning of S2, we immediately see a marked change to Mark. He is now relentless and dogged in trying to understand the nature of this crime, and stops at nothing to be the voice that his son never had.
In S3, we see Mark overcome and overburdened with the immense grief that he still has to deal with. That the case was resolved with an unsatisfactory ruling all the more adds to his sense of personal failing and guilt as a father. In this episode, we still see him trying to wrestle with being so close to his son at the crime scene, yet never having the chance to stop his son from being killed. That gnaws at him and stops him from moving on with his life.
Andrew Buchan displays the full range of a grieving parent to perfection, with tears in his eyes imaging what a conversation with his dead son would be like, to rage and resignation at hearing the guilty Joe Miller confess to his crime.
I feel like you can write an entire separate series on the journey that this Mark Latimer character has had to travel, and Andrew Buchan would be perfect for it.