Following repair work, my connection was stuck at 23.6Mb for 10 days until an engineer visited to request a DLM re-set while on site.
The re-set gave me a 35.3Mb sync speed, which is about the best my connection can achieve.
I had to keep pushing to get the re-set arranged though because:-
a) We are not supposed to be able to see our stats, instead relying on BT's IP Profile (which gives no indication of attainable speeds)
b) The fault threshold speeds are ridiculously low anyway (something like 12Mb or 15Mb for the 40Mb service & I'm not sure what it is for the 80Mb service but you can bet it will be less than 40Mb).