Hi
They would only change the phone if it was within 14 calendar days of owning the phone.
Have you Tried a hard reset.
Please make a back up as this will delete saved data in your phones memory.
Sony Xperia Z2 hard reset
Reset your Sony Xperia Z2 D6502 Android phone
Restarting and resetting
You can force your device to shut down in situations where it stops responding or won't restart normally. No settings or personal data get deleted. You can also reset your device to its original factory settings, with or without deleting all of your personal data. This action is sometimes necessary if your device stops functioning properly.
Your device may fail to restart if the battery level is low. Connect your device to a charger and try restarting again.
To force the device to shut down
1 Detach the cover of the micro SIM card slot.
2 Using the tip of a pen, paper clip or a similar object, press and hold down the OFF button until the device shuts down.
Do not use overly sharp objects that could damage the OFF button.
sony xperia z2 force reset
To perform a factory data reset
To avoid permanent damage to your device, do not restart your device while a reset procedure is underway.
Before you start, make sure to back up any important data that is saved on the internal memory of your device to a memory card or other non-internal
memory.
If your phone is responding to screen input, then open Settings from the Applications menu. Tap Backup & reset > Factory data reset, click Reset phone and then Erase everything to format your phone.
Your device freezes or has fatal errors
If your device freezes or hangs, you may need to close programs or reset the device to regain functionality. If your
device is frozen and unresponsive, press and hold the Power key for 10-15 seconds. The device will reboot automatically.
While using your device, be sure to back up important data. When disposing of your device, back up all data and then reset your device to prevent misuse of your personal information.
THe manufacturers warranty is 2 years so if the above does not work or any other tips you may find.
So you you could ask Vodafone to send it to their 3rd party repair centre.
Or id recommended getting in touch with sonys UK support page for help.
Edit to add link
http://support.sonymobile.com/gb/contactUs/
Current Phone >
Samsung Galaxy s²³ Ultra 512gb Phantom Black.