When I have been upgrading/downgrading from 1.1.1 to 1.1.2, I had to downgrade the baseband so I used this method and it worked great:
Downgrade the baseband firmware:
1. Set Settings → General → Auto Lock → Never
2. Start Installer and install BSD SubSystem found in System Category.
3. Go to Sources and tap edit and add http://i.unlock.no/
4. Now install "Baseband Downgrader" found in Unlocking Tools category. This will take around 5 minutes.
5. When done, restore your phone to get 1.1.1 firmware reinstalled.
6. When phone have been restored, you can Activate and unlock as normal.
Is this not possible with 1.1.3? I assume this would not work for some reason?