The Jargon File (version 4.4.7, 29 Dec 2003):

lord high fixer
 n.

    [primarily British, from Gilbert & Sullivan's ?lord high executioner?] The
    person in an organization who knows the most about some aspect of a system.
    See wizard.


The Free On-line Dictionary of Computing (30 December 2018):

lord high fixer

   [Primarily British, from Gilbert & Sullivan's "Lord High
   Executioner"] The person in an organisation who knows the most
   about some aspect of a system.  See wizard.

   [Jargon File]