BT now produce a device called i-Plate which improves performance and stability on broadband lines by managing the noise and electrical properties from the client end of the line; previously this was only managed from the other end of the line (telephone exchange). The results are described as "gobsmacking" (Read
review in PC Pro)
Here's the actual BT Wholesale documents:
Overview
Technical document
Step-by-step install guide
Note: Sadly the i-Plate will NOT work with some sockets - please read install guide above.
Where to buy:
We do not endorse any particular supplier, but in order to be helpful here is just one example of where to obtain the i-Plate:
http://www.broadbandbuyer.co.uk/Shop/Search/SearchResults.asp?QSearch=bt%20i%20plate
Other important information about line stabilization
Now that BT Wholesale do not provide a "long line" specific product (minimum order level is Max ADSL) they have also introduced a special Service Option that can be added to a line. Here's the official BT description:
"Service Option 3 (Maximum Available Stability)
This variant provides the potential for increased stability over Service
Options 1 and 2 with some reduction in line rate. This service has
different Dynamic Line Management threshold settings to Service Options
1 and 2 to allow this option to be provided. This option offers the
greatest stability for the current BT IPstream Max and Max Premium
products.
Note: Noise Margin can vary depending on the CPE used by users.
Consequently this may impact the stability experienced under a service
option."
...and with regards to 'long line' here's what BT's trial reported:
"improvements in broadband performance over 'long lines'." - read BT press release.
As you can see by the note, having an i-Plate fitted to the customer socket seems a minimum requirement in order for BT to apply this Service Option 3 on a line.
How to request "Service Option 3 (Maximum Available Stability)": please contact us by raising a Support ticket with the subject "Service Option 3" and ask for the product to be added to your portfolio; you can then migrate existing users to this product, or place new orders. Please note that due to being weighted for stability over speed such lines cannot be guaranteed to work with VoIP telephone services.
Final verdict on 'long lines': use a BT i-Plate together with "Service Option 3 (Maximum Available Stability)" for the best possible chance of stable broadband.