Well, for those that are interested in this.
I got an HTC One today. I went into the O2 shop and said I was going to leave unless they matched the internet offer I'd printed off. (600 minutes, unlimited texts, 750MB Data, £30 per month, free handset).
They eventually agreed to £32, free handset, unlimited minutes, unlimited texts, 1GB data. Happy with that.
The HTC One is a nice phone. Dead fast, good to hold and I'm starting to get it set up how I want. Battery life already appears to be absolutely pants though. It was fully charged at 4pm and was almost empty at 10pm with a bit of texting and 30 minutes of talking. Dunno, need to keep an eye on that.