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I'm sorry to hear that, you need to stay in contact with your Local support and the dpd carrier, if your coworkers saw no damage on the phone when you place it in the package and if your Local support said there was damage, you know what happened here, but there isn't much anyone could suggest here, since this is a user based forum.
If the phone is under warranty, why would they charge you £22 to have your phone returned?
@FrontierGate, While processes may differ between regions, the way I understand it is this: The repair centre received a device with physical damage. As physical damage is not covered by the conditions of warranty, the warranty was declined. The £22 would be for an assessment fee for the cost and time of taking a look at the handset and/or shipping costs to return it to the user.
@notveryhappywithsony, I'm really sorry to hear about your experience with this. As uliwooly said, this is a user forum and you will have to take this up between the postal service and your local Sony support team. It would be in the best interest of the repair agent to cover a device under warranty as Sony would pay them for the repair, so I highly doubt they would have damaged the handset and charged you for it. As they can only assess a device in the condition it was received they can not make assumptions as to how the damage occurred and can only follow the process for your local warranty. Good luck with this!