Thanks for this answer, James.
After I joined Zen my sync rate was absolutely stable at 8128 mbps. Then, last November, I started getting a high number of line drops. Customer services suggested that it may be due to BT work being carried out locally or it could be a router issue. There was no reported works being carried out at my exchange. After several days the line drops stopped but returned about 2 weeks later. At that point I decided to try another router and got myself a SpeedTouch 585 v6. Things did stabilise, but by then my sync rate had fallen to about 5500 mpbs. It has now crawled back to 7,648, where it has been for the last couple of months.
According to the SpeedTouch logs, I'm still getting occasional problems. For example, my current DSL Connection uptime reads 44 days whereas my Internet Services uptime is 8 days. I assume this indicates a loss of sync?
Your explanation suggests to me that BT were the cause of my problems all along but getting them to admit to it is another story!
Fred