Strange roaring noise & lack of power
 

Hiya there, perhaps you kind folks can help me...

A few days ago i noticed my mini had a lot less power than i was used to.

At first i thought it was the air conditioning sapping a bit of the power, but even then it seemed like quite a drop in power. The car is making a strange whiney revving noise, (like a scooter or something). I tried to get up a bigish hill, which i would usually be able to easily speed up at 80kmph, but i was struggling to get it up at 50kmph! As this is happening, the air con would come on or go off randomly, regardless of whether its turned on or off. It has been very hot here (43 degrees celsius) but would the heat really effect the power of the car?

I took it into BMW, the guy thought nothing much of the loud revving noise (i might be a young woman, but im not a fool and i know when my car isn't driving the way it should be) and asked me if i was using premium petrol (i am).
My car is an automatic, but I have been reading lots about transmission problems with the mini, mine was in the shop a few months ago with the power steering failure, luckily its still under warranty so it didn't cost me anything to fix. Its still under warranty.
Does this sound like a common problem?
any tips on what i can ask them to look into when it goes into the shop next week?
I really dont want to think i have bought a lemon, my dad tried to talk me into getting a Hyundai Getz (also sold as the Hyundai Click or Hyundai TB).
was he right?

Thanks!