Router ASUS-RT-AC66U

Although previously unable to get this router working with FTTC, a newer (standard) firmware from Asus (3.0.0.4.374_130) has options that enable it to be set up to work along with IPv6.

Connect the BT openreach VDSL modem LAN1 port to the WAN port of the router.

On the WAN page, the WAN connection type can be set as PPPoE, and the user/password can be entered in the account setting. (I also set the 'Enable VPN + DHCP Connection' to Yes as well, but don't think this is mandatory to get it working).

On the NAT Passthrough page, setting the 'Enable PPPoE Relay' to Enable and apply these settings.

On the IPv6 settings section tab, this can now be set to 'Native with DHCP-PD' which retrieves the details directly without any further setup providing the correct LAN IPv6 Prefix and Length and correct LAN address.