Did you sort your problem out?
From reading your thread am I correct that you have
1. created a share [say its called sites] on your Duo
2. you have re-directed web access on your Duo to this share.
3. within the 123-reg control panel you have re-directed your domain name to point to the share on your Duo
This might be http://xx.xx.xxx.xxx/sites
Is this the method you used?
I use this method and it works for me.
The xx.xx.xxx.xxx is of course your external ip address.
NAS: Readynas Pro 6 - 3x2TB [ST32000542AS], ReadyNas Duo - 2x1TB [ST31000528AS]
Network: Airport Extreme [Gigabit Ethernet / Wireless], Western Digital 2TB, Virgin Media Super Hub [30Mbps]
Hardware: 24" iMac, 13" MacBook, 11" MacBook Air, 2 x Apple TV2's, iPad 1, iPhone 3G
Clients: OSX 10.7.2, IOS 5.0/4.3, Plex 9.3.4
Music: 1 x Logitech SqueezeBox Classic, 2 x Logitech SqueezeBox Boom's, M-Audio Oxygen 88, Apogee Duet 2
Software: Adobe CS4, Aperture 3.2, Guitar Pro 6, Logic Pro 9, Vectorworks 2009