I am looking at getting the new 2860Ln and using it with Infinity.. Firstly does this work ? I assume I just replace my BT HUB 5 with this and its good to go

Secondly are there any guides on how to set this up, I dont beleive infinity router has a username and password on it .

You don't use the Infinity router if you get the Vigor2860Ln and yes, it is compatible.
It's easy to setup and depending on your ISP there is a username/password but
there are setup guides or support can help you.