How do i approch the subject of a pay increase?

[babyface29]

I started in my job about 5 months ago and the annual salary review is coming up. Two of us started in the same positions. Since then i have taken on something else aswell as doing the job to my spec, which is classed as a higher 'per day' rate, not dodgy at all i promise! The other person does not do the other job. Am i making sense? To get 'my case' heard i need to book an appointment with the Director I need to go through the Director's PA, who coincidentaly is my line manager, who was against me taking on this new project. I am just worried that she will get involved and if i mention that because i have taken on the extra i should be compensated for it, instead of giving me a payrise they will just stop me doing it (its something i enjoy and varies my day) and hold the fact that i have been cheeky enough to ask for a pay rise aaginst me.
Help - please can i have some advice about how to go about the whole situation.