For ages, I had a data blackout on a section of my commute to work. A number of queries to O2 & challenges to their unacceptable answers didn’t help.
Over the weekend, I found advice to reset all network settings which seems to have turbo charged my data connection. Even in that blackout section.
If you’re having trouble with data on the O2 network, I recommend resetting your device. On iOS6, you’ll find it under Settings -> General -> Reset -> Reset Network Settings