It's interesting that SC decided not to do the auditions, maybe because he wasn't too bothered about more good acts getting through.
Usually the semis have the 'best' act on last each night. This series it has only happened on one night, the first when Ronan was on and won. I was surprised that they mixed them up, but this theory might explain why.
Finally, Simon said last night that the first time he met Ronan was at the first audition, which was interesting because he wasn't judging so why was he there?