A YouTube video by Brady Haran, Mapping the Meridians - Objectivity 97, ended in a cliff hanger (the answer not found by Brady, and Keith Moore of The Royal Society). It's with regard to the Anglo-French Survey of the late 18th century. From Wikipedia, which does not explain the problem:
In 1783 Cassini de Thury addressed a memoir to the Royal Society in which he expressed grave reservations of the measurements of latitude and longitude which had been undertaken at Greenwich Observatory.
From the citation, Concerning the Latitude and Longitude of the Royal Observatory at Greenwich; With Remarks on a Memorial of the Late M. Cassini de Thury. By the Rev. Nevil Maskelyne, D. D. F. R. S. and Astronomer Royal, I concluded—disclaimer: I'm not knowledgeable enough of the then-problems associated with such surveys, and their astronomical (no pun) implications—of concerns raised regarding the accounting or lack thereof of the refractive effects of the atmosphere on the observations undertaken at Greenwich, and thus its coordinates based on polar stars. Continuing on Wikipedia:
The final report of 1790 presents figures for the distance between Paris and Greenwich as well as the precise latitude, longitude and height of the British triangulation stations.
Given my aforementioned position, I'm at a complete loss in the latter bigger document. Given the above summary on Wikipedia, and how the video ended, I'm guessing the direct answer to my question was not explicitly stated.
Based on comments by @PieterGeerkens: the subsequent movement of the meridian when a new instrument at a new location was used, does not address the earlier error being asked about.
Kindly note: I'm asking about what was found in 1790, not what was found in the 20th century using "a new triangulation".