Hard to tell from the public Chorus map, but at a guess there's no active ADSL cabinets around the outskirts of Oxford. There's a central core of VDSL coverage with a corresponding larger area of ADSL (on longer lines obviously). If that's the case, unlikely to be a conklin, just a very long line.
Really need to know if the sync rate and noise margin change at night. If the noise margin drop, and the sync rate goes with it, then there is some sort of interference - could even be from other lines in the same feeder at that distance. If the sync rate and noise margin remain unchanged, there's some other congestion.
Chorus have been publishing usage graphs periodically during the last few weeks - they quite clearly show the daily high demand times, and the evening is a peak when people are streaming entertainment.