It's true and it isn't...trouble is, you will alway's be restricted by copper wire connection, which can be degraded by corrosion, water wind etc.
I have the same issues, but manage a steady 2.3 mb that is as good as it gets when you live in the country!
As for Plusnets comments, they are correct to refer you to BT as they maintain the line.
You can phone the engineers and ask them to boost the gain on your line (not sure if this still works, but I know a local engineer)
If your speed continues to degrade, then I would get back to plusnet.