I believe as I have a FTTP setup through BT and therefore have a fibre box on the wall, I can just get a RT-ACxx model as opposed to DSL-ACxx but just looking for comfirmation this is the case
Yep, the ONT is a modem so you don't need a modem/router just the router (RT) model.
Any ideas what will need to be done config wise on the Asus? or will it just jump to life when plugged into the Modem?
I haven't got an ASUS so can't tell you the default config, but you will definitely need to enter PPP username of firstname.lastname@example.org as username and bt as password when prompted.