Have you documented all that you do and made note of your achievements? I ask because when you do enquire about a pay raise you need to accompany that request with relevant proof of why you are owed it. If you haven't started already, make a list of all the tasks you perform in addition to the Terms of Reference that are already in place (you ought to have signed one before starting the jobs). If you don't have one, get one. Request they formalize and put one together for you. That list you are making then should include those tasks and performance objectives that go beyond the scope of the Terms of Reference. Make note of your achievements. It is really important now to provide examples for each point you are making by which you believe you are owed increased remuneration. Basically back up, back up, back up every point you make.

Great luck to you!