@katty59: Sorry to hear of your experience with O2. Did you reach out to inform them that you intended on returning the phone prior to sending it back, as they may have a returns process that needed to be followed. Did you use returns packaging provided by O2? I'd recommend reaching out to them to clear this up if you're still inside the 14 day cool off period, just to make sure that everything is in order, as you may end up incurring a charge if they don't have a record of the returned device.
@katty59: I'm not familiar with O2's returns process, but I definitely recommend reaching out to them to let them know that you have returned your phone, as I assume they will need a record of this.