Our 2005 Cooper S with air con and manual trans uses Dayco #5060548. That's the only one listed for an 05 S with Advance Autoparts and Autozone as well, and the number code decodes as a 6 rib belt that is 54.8" in circumference.
Be aware that a proprietary tool is recommended to release the tensioner and free the belt. The tool is the easiest way to get it done, but at over $100 online, it sure isn't cheap. I changed ours by carefully using a prybar. It did cost a little lost paint on an underhood brace, but it can be done. Also, the tensioner uses a really strong spring because of the supercharger, so be sure to pin the tensioner back securely using the provided hole - it could be dangerous to errant fingers if it comes loose.
Good Luck
david