Hi, I think you are right to mention all what you do for the practice, late nights, weekends and extra duties, sometimes they need pointing out, also if you have done any courses that will help also, and the fact you have not had a pay rise in 2 years, its also a good idea, to mention any further courses you would be interested in doing, shows initiative. And that you are willing to invest your time in the practice.
You should not be on a trainee's wage now, you have been qualified for 4 years, make a note of what you want to say, practice it and be confident, make him see you mean business! As to what nurses earn in private practice, sorry I work for a mixed NHS and private, and would be very interested my self hahaha Good luck!