Thank you for the welcome and for the constructive response.
I thought I indicated Hardware/Firmware versions at the start of my post (DSL-320B Z1, 1.05) what more information can I give? As for region it is UK.
I do not have an ISP modem, I am currently using the DSL-320B as my DSL modem in PPPoA/PPPoE mode.
These errors occur consistently, including periods when my download speeds are high (>9 Mbs), my ISP is not overly helpful, does not think I have a problem and does not want to undergo the expense of a physical line check.
I have no means of ruling out a filtering problem, but I have experimented today with swapping over my DLINK filters with other high quality ones which I have, it seems to make no difference.
You have pointed to some potential causes of this message, I am still interested to know a bit more about what it actually means. Is there some kind of document listing error codes? I am also wondering if there is something in the low level DSL settings that can be adjusted.