Well my Isp is suggesting that we get Bt into look at the problem, and believes it is noise on the line.
Although I have done the quiet line test at the point whereby the noise starts to cause the Snr to drop and there is no noticeable noise on the line.
Im at a loss other than to suggest that something somewhere is causing the interference but that i'll probably never find it.
I have tested:
1, The router plugged directly into the master socket via the filtered faceplate, no telephone attached = same noise levels
2, The router plugged into a separate adsl filter which in turn is plugged into the Bt master Test socket = same noise levels but drop in sync
3, Moved the router to different positions
The only things I can see that are left are the router itself, or noise created outside of my property i.e. somewhere on the line from the exchange.