hope someone can advise me.I dont use my landline at all now as i use a mobile.i have b t infinity ,can i cancel my landline and just use my broadband from bt,

No.

The broadband is delivered over the landline. Though in the case of Infinity it is only the last bit from whatever roadside cabinet you are connected to that actually carries the broadband. The bit going back to the exchange just carries the phone service.

If you had ADSL then the entire line back to the exchange would carry the broadband.

Unfortunately however in the case of Infinity BT don't let you just rent the bit of copper wire between your home and the cabinet, you have to have the bit back to the exchange whether you use it or not. And even if they did let you rent just the last leg of the connection I doubt they would charge much less. Bearing in mind they charge everyone the same rate now whether you live 10 feet or 10miles from the exchange.