Thanks for the feedback everyone. It isn't that I think £15 is ridiculous or anything like that, but with the problems in the flat it might be somewhat unfair for the moment. The rent was already high to begin with for Dundee. Also, when we moved in the agent made it sound like we would be keeping the rent where it was for a while because we said we plan on staying there for a long time. It worries me a little that they seem to be raising the rent regardless of this and the problems that have arisen.
The other thing is, I know that I can't be kicked out for having a baby. I never said I thought they would blatantly do that. I was worried that they would find out and then raise the rent beyond my means in the hope of getting me to move. There is no way I could prove this was because of the baby so they could easily have gotten away with it. Now that I have this rent increase in writing before I have informed them, though, I'm not so worried about the side of things.
