Call O2.
When I bought my 3GS (and transferred over my number from 3G) I still had a couple months of the Web and Wifi to run (from the 12 months that came with the 3G). I started getting messages that they intended to take £10 when that ran out. I called O2 and they activated the 12 months' Web and Wifi Bolt On that came with 3GS. They had me fax over the 3GS receipt.