The fault is periodic lock-ups of the ADSL service, PPP fails but DSL stays in sync, the fault only clears on a manual re-train either by disconnecting cable, soft re-sync from router GUI or DSLAM port reset. BT is blaming an electric fence along which the 50 pair cable passes, even though (1) we've seen the lock up when no errors were being detected, (2) the fence was in place in exactly the same form for two years before this fault started and (3) there is no evidence that any other service is affected by the same fault. This last point gets confused because I can well believe other properties see errors, but if DLM is left to it's own devices these errors are resolved by deeper levels of interleaving. What I can't believe is that all the other users are having to reboot their routers so frequently but have not raised this as a fault.
I did some checks on the fence and I'm pretty sure I could make big reductions in the errors with a bit of time and money, but have a nasty feeling that even if I did so BT will still use the same excuse. One or the OR guys was claiming that even if my router and the DSLAM didn't see errors then they would nevertheless still be present. The fact is there's no proven cause/effect link, because unfortunately the owner can't leave the fence off for days at a time.