I swapped from a bridge adsl modem setup (the connection is defined and terminates as ppp0 on my Linux box) to a ADSL Router setup where the username and password is in the ADSL Router itself.
The problem was that my Linux box still had the pppoe connection defined and started on boot. I used pppoeconf to configure it but there didn't seem to be an option in pppoeconf to delete it once configured.
The fix is open /etc/network/interfaces and comment out the entry as shown
auto lo
iface lo inet loopback
#auto dsl-provider
#iface dsl-provider inet ppp
#pre-up /sbin/ifconfig eth0 up # line maintained by pppoeconf
#provider dsl-provider
iface eth0 inet static
address 10.244.139.25
netmask 255.255.255.0
gateway 10.244.139.253
auto eth0
Although this worked for me, the eth0 was not present in networkmanager anymore (after a fubar pppoe conf)
Movin the /etc/network/interfaces file to /tmp and rebooting fixed everything for me.
Thank's
Nick