iPhone 3GS- Cellular Data Network - Cellular Data APN allowing Internet

I bought an iPhone recently, and in the instructions it tells me to change something to make sure i'm on Pay and Go, which will then allow me to use 3G, Edge and Spotlight internet connections. I have to do this: Settings > General > Network > Cellular Data Network > APN box > type in "payandgo.o2.co.uk" changing it from "mobile.02.co.uk". It didn't say "mobile.02.co.uk" when I first went on, it said "idata.o2.co.uk", so I wonder if that had anything to do with my situation. Anyway, when I changed it from that to "payandgo.o2.co.uk", the internet only worked for about a day. So I checked if it still said "payandgo.o2.co.uk" in the APN box, and it had changed back to "idata.o2.co.uk". Now whenever I try to change it to "payandgo.o2.co.uk", and go back out of settings, it immediately goes back to "idata.o2.co.uk". I've tried exiting the settings in different ways to see if that would change things but it doesn't. Now none of the Edge, 3G, or Spotlight Internet connections can be used, even though they appear. What can I do to fix things?