Y. Shu, S. Koposov, N. Evans, V. Belokurov, R. McMahon, M. Auger, and C. Lemon. (2019)cite arxiv:1909.02010Comment: 18 pages+appendice, 11 figures, 2 tables, accepted by MNRAS. The C75 AGN catalogue can be downloaded at https://www.ast.cam.ac.uk/~ypshu/AGN_Catalogues.html.