sounds right.
most providers now provide a router that can be set up to use the engineer installed openreach modern, or plug into the phone line filter. No need for engineer install these days.
edit: To be honest, you may be able to plug it into the master socket as on the right hand option here and remove the openreach modem, but I don't know where the broadband signal is filtered off when you have an openreach modem,, before or after the master socket?!
http://help2.talktalk.co.uk/guided-assist/how-setup-and-check-my-fibre-equipmentguess you can try 2 (right hand), and if that doesn't work revert to 1 (left hand)!!