My issues are all related to unlocking the bootloader... (I think) - I can't upload pictures @ work, so I'll type away below:

F5121

Rooting status: Bootloader unlock allowed: Yes

I'm running 34.3.A.0.238

Developer options: ON
OEM unlocking: ON
USB Debugging: ON
Verify apps over USB: ON

Select USB Configuration: MTP (Media Transfer Protocol)

Everything else (in developer options) is OFF

Location: OFF

Lock screen & security:
Protection by my Xperia: OFF (this was on previously)

Device administration:
Unknown sources: ON

____________________

From the Sony website (where the instructions don't match either the video or the links provided) I have searched elsewhere & downloaded (as shown on the Sony video) installer_r24.4.1-windows.exe

I've downloaded the fastboot driver

I've (eventually) got it running in Windows 8.1 (rebooted into Testmode & run unsigned driver, again these steps don't match the Sony steps in the video

I've got the CLI running

I've typed in fastboot devices

a code has returned (I don't have it in front of me)

I've then (from step 4/4 on the unlock your bootloader Sony website) entered (Copy pasted) the code

and FAILED is al that happens...