As I don't have a syslog server or an always-on computer to run one on, I have configured the System Log to be saved locally. It seems the log is cleared automatically every day at 0.00.
This is not terribly helpful. Is there any way it can be configured to store log entries on a fifo (first in, first out) basis, or similar? The quoted memory usage on the router is around 60% so it's not as if there isn't room for more log data.
I also struggle to find anything in the sytem log relating to VDSL disconnections but maybe that's for another day.