Well, I phoned O2 this evening and I was very impressed by the lady I spoke to. She seemed to generally want to help me rather than my store who just seemed to want me to go away. We talked for a while about my options, she consulted her manager a couple of times but the long and the short of it was that they couldn't help because it was store bought. She said had I bought it online they would have swapped it out.
One thing she did do was point me at
Rental from O2. Which I didn't know existed. You can rent an XDA2i for £15 a week.
I think it was the first positive experience from O2 customer service. Shame they still couldn't help really.
Back to my crappy store I guess.