What they said.
Home networking (phone extensions and the like) can introduce all manner of nasties into the signal. Shouldn’t apply in any meaningful manner if you’re in the Test Socket unless you work in a power station, clue’s in the name.
Regular DSL goes through a learning process called “training” in the first couple of weeks to work out what speed it can reliably connect at. Turning off the router during this process can be interpreted at the ISP end as “oh, the line dropped, I’d better slow down.” Once training has competed this sets the speed at which it will try to connect.
That said, if this has happened they can simply restart the training process again, saying your line’s slow because you turned your router off is mince.
Where did you get a one inch ADSL cable from?