It's been weeks (or months?) that this feature hasn't worked. I've tried from many different computers in 3 or 4 different locations and ISPs, it always comes up as an endless redirect loop. Some browsers just let it run on forever, but Firefox and Chrome pick it up and return the relevant error message.
I've emailed O2 Customer Support about this, and all they said is that it works fine for them and asked me for my system configuration and to clear my cookies. When I told them I'd tried it from 4 different computers/locations, they said ring up customer services!!! Can't afford the time to twist their arm to fix this, but as I see more people reporting it maybe we should all send emails to them together - perhaps this will wake them up.
That said, a free text service is probably not something they would go out of their way to provide