The returns/swap policy seems very hit and miss.

I tried getting a new Pre for my Oreo Pre at the O2 store on the Strand, London. It was the worst O2 shop I've been in, horrendous customer service and they flatly refused to do anything about the Pre. Frankly they didn't care. After lots of arguing (and me asking for their regional manager, etc) they offered to send it off for repair (refused to say where!) meaning I would be Pre-less for 10 working days.

Next I tried at the Liverpool branch the O2 was bought from. They also refused but offered the sending off for repair option pretty sharpish.

Finally I tried another O2 store this evening in North West London and they offered to swap the Pre for a brand new one, an offer I accepted. No questions asked. All they wanted was confirmation of the issue which they got to see first hand.

Moral: if one O2 store doesn't help, try another. They don't seem to have a nationwide policy on this.

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