When will I get confirmation of second choice?

Hi, folks.

I'm in a bit of an odd situation and am not too sure about how to proceed. Basically, I've got two offers for positions in different universities. My backup choice (Which has become my prefered choice since attending a summer school at the backup university) is an unconditional offer. My main choice had conditions which I know that I have not met (it required that I studied an extra course, which I was unable to do).

The situation I have ended up in is as follows; I have not met the conditions for my main offer, but have an unconditional backup offer. I have long since changed my mind about which university I would prefer to go to, although it is obviously far too late to change. My concern is that my main offer has still not been turned down on UCAS, and so I can't actually be one hundred percent sure whether I will get in to my backup choice! Have universities not begun to turn down offers yet? What if it reaches the start date of my backup choice and I have not yet been rejected from my main offer?

My main choice (the one for which I have not met the conditions, and do not want to go to anyway) begins on the second week of September, where as my backup choice begins on the third. It's seems a little late to me to be waiting for full confirmation.

Many thanks.