To do this, there are two network configuration settings which need to be changed. The Hub assumes that as it will be the central network device, it should be responsible for handing out IP addresses to clients on the network via DHCP, on an IP subnet which can be configured on the box.
Firstly, DHCP needs to be disabled, as this will still be handled by the DG834. Secondly, the Hub needs to be on the same IP subnet as the DG834. In this case, it was different, so a change was required. By default, the IP address used by the Hub will be, for example, 192.168.1.254 and it's also worth checking that this won't clash with anything else on your network.
My DG834 is configured to use the 192.168.3.x address range (for historical reasons), so I needed to make this change to the Hub, in other cases, this might not be necessary.
To make these changes, access the Home Hub via the browser interface by connecting directly to it using a LAN cable (a laptop will come in handy for this).
Select "advanced" and then continue from the warning screen. You'll need to login at this point with the Hub admin username and password. Then select "IP Addresses":
Once reconfigured, you can connect the Hub to your existing broadband router using an Ethernet connection. I did this using Powerline Adapters, which allowed me to locate the access point at the back of the house, to give a stronger signal in the garden.
Here's a summary network diagram: