OK bear in mind I'm not exactly a technical networking expert, I've just got it to work and there may be much better ways of achieving the same. Once the line has settled I'm going to mothball the HH5 as an emergency back up.
The HH5 is set up with the IP range 192.168.2.xxx, the HH5 itself is assigned 192.168.2.254 apart from that it's configured out of the box from BT. So for this example you need to make sure your modem IP is set the same.
The DSL-AC68U is connected to the HH5 using a standard LAN cable plugged into a normal gigabit ethernet port on the HH5 (not the WAN port) that is then plugged into LAN Port 1 on the Asus.
Config for DSL-AC68U:
LAN > LANIP: IP Address: 192.168.1.1, Subnet Mask: 255.255.255.0
LAN > DHCP Server: Enable the DHCP Server: Yes, Domain Name: Blank (as in nothing) IP Pool Starting Address: 192.168.1.2, IP Pool Ending Address: 192.168.1.254, Lease time: 86400, Default Gateway: Blank, DNS Server: 192.168.2.254, WINS Server: Blank.
WAN> DUAL WAN: Enable Dual WAN: ON, Primary WAN: Ethernet WAN LAN Port 1 (important it's the same port that you plugged the cable into on the Asus), rest of settings not important.
WAN> Internet Connection: WAN Type: Ethernet WAN, WAN Connection Type: Static IP, Enable WAN: Yes, Enable NAT: Yes, Enable UPnP: Yes, WAN IP Setting: IP Address: 192.168.2.253, Subnet Mask: 255.255.255.0, Default Gateway: 192.168.2.254 (i.e. the IP of the HH5), WAN DNS Setting: DNS Server 1: 192.168.2.254, DNS Server 2: Blank, Account Settings: Authentication: None, Special Requirement from ISP: Host Name: Blank, MAC Address: Blank, Extend the TTL Value: No
Obviously you need to apply the settings after each screen, once it's all done go to Network Map and the Primary WAN should say connected. You can test the Internet Connectivity on the router itself using the Network Tools menu. Hopefully you can now connect a client to the Asus and get an internet connection.
Now need to duck and wait for all the comments about not doing it like that etc.