From looking at the SNRM plot it seems, to me, that there is a defective joint. Whether it is a defective joint in the D-side, the PCP to fibre cabinet tie-cables or the harness, from the Krone blocks in the fibre cabinet to the connectors on the DSLAM line-card, I cannot say.
It is examples like the attached, below, which should be shared with Plusnet so that they, ultimately, could pass such information onto Openreach.