There are a lot of articles on the Internet suggesting that it is impossible to leave a VirginMedia SuperHub in router mode and use Dynamic DDNS. They are plain wrong. If you want, for example, to use OpenDNS content filtering then you can.
Provided that you have a second router capable of running DD-WRT, it is straightforward.
Once you have DD-WRT installed on the other router, you just need to ensure that DD-WRT is configured to check the external IP address, not the WAN IP address:
Connect the WAN port of the DD-WRT router to one of the Ethernet ports on the SuperHub and that's it - DDNS is now up and running but you can still use the features of the SuperHub router, such as guest wireless, and any devices you connect to the DD-WRT router have the extra protection of both the SuperHub and DD-WRT firewalls.