After finding it a bit unreliable, I had a poke around and found that if I telnet it , I could change the DNS settings, which seems to make it better. At the telnet prompt type
dns config static 126.96.36.199 188.8.131.52
which sets the box to use google and freedns. The system used dnsmasq and gives out the lan ip as the dns server to it's clients, which works fine.
dyndns was more interesting, because at the telnet prompt you have a dyndns option, but I couldnt get it to work, so a poker around the various web pages on the box , I found this
and set it up , though had to choose LAN. once done, I backedup the config under management-settings-backup , opened the backup file and changed the interface from br0 to ppp_255_65535_1 which is the wan interface (find it with ifconfig, though I think this is universal). Then restore the config and it worked fine.