On my small 1960's Dormer, we removed Tiles then we decided to keep it simple and filled the void in the Studding with Celotex, then OSB, followed by a DPM/Vapour Barrier, then vertical battens and then finished with Marley Boards. We could have re-fitted horizontal battens and Tiles but prefer...
You might think of the cheek walls as a flat roof as they are similar in construction? Research "Warm Roof" construction, ie, the stud wall is left free for ventilation and an insulating layer is added to the outside. Maybe clad the walls on the outside with 6mm ply, a vapour barrier and then...