British customers often open a complaint with an apology. The words are mild, but the complaint is serious.
Sorry to be a pain, but: There is a problem, and I expect it to be fixed.
I don’t want to make a fuss, but: I am making a complaint.
It’s probably just me, but: This is wrong.
I’m not entirely happy with: I am unhappy with.
Would it be at all possible to: Please do this.
When you get a chance: Soon.