"Depuis, c'est le chef de l'opposition à l'Assemblée."
I think what sitesurf says previously clears that up. Because it is prefaced with a duration (depuis=since then) it is "he has been". The previous use of c'est may not have been in relation to a duration of time.
That was already discussed on another thread and I seem to remember that the outcome was that both "il est" or "c'est" should be acceptable, because of "THE chief of the opposition" implies a single position/title.
In other words: - he is a chief = c'est un chef - he is the chief = il est le chef, or c'est le chef.