This device cannot be factory reset by pressing a combination of keys. The reason for this is because they do not have the android recovery mode installed by default.
If you need to recover your device and can't get into the menu system, then your best option is to use the PC Companion to do a phone repair.
Connect your phone to the PC using the USB cable supplied with the phone and start up the PC Companion software, you can find the option to repair your phone under Support Zone > Phone / Tablet Software Update > repair my phone / tablet.
Note: If you haven't yet installed PC Companion on your PC, you can download it from www.sonymobile.com/gb/tools/pc-companion/