Why does my sony ericsson xperia mini pro tesco mobile let me on the internet free?

Is it really free?

I'm pay as u go, with Tesco (UK) . I checked my balance, have gone on the net, and then checked my balance again and it is the same!

Is it perhaps free when I am at home near my broadband connection only. Will it charge me if I log on when I'm away from home?

Please help.