I would like to monitor and record how much bandwidth a particular link is
using, the problem being that the router is managed by an external company,
so I have no access to set up SNMP with MRTG etc.

The router is connected to a hub and a few other devices, so I was thinking
to sniff this network using Ethereal, but does it have a bandwidth monitor
function. If not then what tools do people suggest to accomplish this task.