These are the errors I found in my router's system log...
Sep 17 17:05:57 daemon pppd[1451]: Doing disconnect
Sep 17 17:06:00 daemon pppd[1451]: PPP: Start to connect ...
Sep 17 17:06:00 daemon pppd[1451]: PPP server detected.
Sep 17 17:06:00 daemon pppd[1451]: PPP session established.
Sep 17 17:06:00 daemon pppd[1451]: Using interface pppewan_1
Sep 17 17:06:00 daemon pppd[1451]: Connect: ppp_ewan_1 <--> eth0
Sep 17 17:06:00 daemon pppd[1451]: Couldn't increase MTU to 1500.
Sep 17 17:06:00 daemon pppd[1451]: Couldn't increase MRU to 1500
Sep 17 17:06:00 daemon pppd[1451]: Couldn't increase MRU to 1500
Sep 17 17:06:00 daemon pppd[1451]: PPP LCP UP.
Sep 17 17:06:30 daemon pppd[1451]: Remote message: CHAP authentication failure, unit 5806
Sep 17 17:06:30 daemon pppd[1451]: CHAP authentication failed
Sep 17 17:06:30 daemon pppd[1451]: Couldn't increase MTU to 1500.
Sep 17 17:06:30 daemon pppd[1451]: Couldn't increase MRU to 1500
Sep 17 17:06:30 daemon pppd[1451]: User name and password authentication failed.
Sep 17 17:06:36 daemon pppd[1451]: Connection terminated.
Sep 17 17:06:36 daemon pppd[1451]: Doing disconnect
Sep 17 17:06:39 daemon pppd[1451]: PPP: Start to connect ...
...looping for over three and a half hours at which point everything was back to normal.
Times above are GMT. The problem started at 17:58 BST, and ended at 21:41 BST.
Edited by deleted (Thu 18-Sep-14 16:08:39)