Unhappy O2 UK original Firmware for Galaxy S3

Hi all, I have tried to unlock my galaxy S3 gt i 9300, which is locked to O2 UK. I live in Ireland and bought the phone from a guy on e bay. I got the unlock code from two different sources and both are the same, but neither will unlock the phone, I have tried the unfreeze code too but it gets me nowhere. Entering the keyboard codes does not work either. The firmware baseband version which is on the phone at present is I9300XXUGMK6, the build number is JSS15J.I9300XXUGNA5, I feel that if I can get the phone back to it's original O2 UK firmware then I might be able to unlock the thing. My problem is that I have found I9300XXALE8_I9300O2UALE5_O2U to be the original firmware, but the link to it from every site is dead and I cannot download it, all the links were to hotfile and we all know what became of it. If anyone has any other suggestions as to how I might unlock my phone then I would be more than happy to try them.

Best wishes Wez