Going right back to the original post
I've used Bald_Eagle's tool to download some graphs and they show little to no variation in the bits per tone or SNR over the course of half an hour. Would I be correct in saying that the cause of my current 30Mb/s throughput cap cannot be interference?
It is very common to have interference that is time of day dependent, with sometimes quite extreme differences. So a stable signal over 1/2 hour is not nearly enough to indicate line quality; you need to monitor for at least a couple of days.
Many people who have such interference would like the higher speeds when it is possible, even thought that would force regular disconnects when the noise starts. DLM does not allow you to make such a choice.
A modem log (eg form Bald_Eagle's tool) is an excellent start to diagnosing this. Sometimes it will point to a noise source that can easily be eliminated. Unfortunately, such noise is difficult to track down even when you have an excellent log, and often realistically impossible to fix even if identified.
When I was on BE ADSL I had a regular 18/19Mb connection. At one point, a noise source started from 8:00am to 9:00am each day, reducing my speed to 15Mb or less. After a few months it went away, and I was never able to identify it. At least, as BE didn't use DLM, I could resync after 9:00 when I knew form the stats that the noise had gone away and get my normal speed for the rest of they day if I needed it.
Moved (with trepidation) to BT Infinity 2 for upload speed. Happy BE user for several years.