A number of trigpoints in "North Central England" were retriangulated in the late 1980's. This seems to be related to the (pre-PSD) GPS Stations. If anyone knows more details, please add them here.
An email request to the OS in Nov 2021 for information on this elicited the following response:
'As far as we are aware the NCE was a readjustment of older obs supplemented by some new ones, probably including many more distances than originally and perhaps even some early GPS measurements. We think the aim was to improve the quality of the trig network across the region and to correct some historic problems and also improve coordinates in some areas following decades of mining related subsidence. You may be able to find more information at The National Archives in Kew.'