Almost exactly one year from connection I thought I would give an update. The attainable has dropped from 107Mb/s to 70 mb/s (currently higher as someone seems to have gone away) and is still dropping. The bad news is that most neighbors don't have FTTC so there must be plenty of scope for more drops. The only good news is that the cabinet is 75% full so only 25% to go.
I have retested my various efforts at optimization of the line. Although the two go together my main aim has been to keep errors low rather than get a better speed. The speed is way more than enough for my use and the only thing that has annoyed me is two occasions when interleaving was applied and I actually did notice an impact. I am sure that what is best to reduce errors depends a lot on noise that previals on the line. I have a general tick over rate of erors coupled with what seems to be SHINE events that occur about 3-6 times a day with potentially many CRC and one to several SES each time. As noted below the SHINE and some of the general error count seem be mains borne.
My biggest benefit comes from running the modem and router, mainly a HG612 and Dlink Dir-655, on a BT mains conditioner. The Main conditioner seems best, if contrary to recommendations,it is unearthed!
Another a big benefit is also gained if there is NO extra wired ethernet connection to the modem/router but with all access wireless. I guess this set up is close to running on batteries. This has not proved a disadvantage for me but as a matter of principle I do prefer a wired connection.
A smaller but noticeable benefit comes from using a home made common filter using toroidal ferrites. The does not reduce the background error rate but does help reduce the impact of the SHINE like events.
I was previously earthing the unused drop wires but with the new set up that hardly makes a difference and those earth connection been disconnected.
When compared to a straight mains connection - and using just the mains conditioner, the SES rate, a good indicator of SHINE, drops from ~6-10/day to ~1-2/day. With both the conditioner and wireless access only, the SES rate drops to ~0.5/day. With both improvements in place the overall ES/day is also reduced but by only about 30-50%.