This could be used with all Xperia phones running android.
I have used this method starting with Xperia X10a, Xperia S and now Xperia Z1.
1) You have to use Sony Update Service or PC Companion to repair/reinstall your software. You will loose all your data!!! So make a back up of your content.
2) After that done go to C:\Program Files\Sony Mobile\Update Service\db\13740270\blob_fs
and you will see 4 files with name starting FILE_###########, where # - is number that can be anything.
Make sure that they have similar date and time creation. If you had used SUS before,just delete folder db to make sure there no other files from previous updates, because it will screw your phone up.
3) Copy those 4 files to a folder you name anything you like somewhere on desktop for easy locating it later on.
4) Then go on and install latest Flashtool.
5) Once it is intalled, just open Flashtool, navigate to Tools tab, select SEUS decrypt. Now for source folder navigate to those 4 files you have previously copied. Four files should appear on left side, highlight them and press "->" , Now those 4 files should move to right side. To complete this step press Convert.
6) In status window you can see what is happening. It can take some time depending on your PC.
7) New window will pop up once decrypting is done.
8) This step will create FTF bundle. Fill up information on Device, Branding, and Version.
9) Move all files from left box to the right (highlight and press ->) and then press Create.You can take a 5 min break.
10) After Flashtool finished bundle creation go to C:\Flashtool\firmwares to locate FTF back up you have made.
Hope that helps!
If you have any further questions just ask, I will do my best to answer it.