Since then the voice line has remained quiet and the broadband performance stable but degraded compared with its previous excellent state. My ISP tells me that its current performance is within the expected performance band, so I will not be pushing it any further.
It might take a while for the DLM to readjust your profile now that the RFI source has been removed. If your ISP is really understandable, they should be prepared to reset the line and allow it to "train", just as if it is a new install. Perhaps you need to communicate with a "higher level"?