Samsung have released a firmware fix for the unbranded UK version I9300. This update is only 1 MB in size. It contains a modem only. According to Samsung it contains a fix for the dreaded battery drain experienced by most Galaxy S III users. The claim once installed the battery drain will be completely gone.
If you live in the UK you can update your device now over OTA or KIES.
If you have custom firmware installed on your device you can update via ODIN. This update will not cause any problems with the root or the custom Rom due to being Modem only update.
You can download the firmware below and flash it via CWM.