I have a SIM free 9700 and I'm on O2 Simplicity 20 (600 minutes, 1200 SMS and unlimited internet). You need to tell O2 that you have a BlackBerry and they will set uoiu up with the BlackBerry Internet Service (BIS), accessed here
BlackBerry Internet Service. Once O2 has done its stuff, login here and set up your phone and email.
DO NOT TAKE NO for an answer from O2 - really. They may talk to you about a BlackBerry Bolt-on. I got this for free 'cuz O2 said it came whenever a BlackBerry was connected. Mrs' Pete6 got the same deal.