The Ministers and Chiefs of the Defence Staff are supported by a number of civilian, scientific and professional military advisors. The Permanent Under-Secretary of State for Defence (generally known as the Permanent Secretary) is the senior civil servant at the MoD. His or her role is to ensure the MoD operates effectively as a department of the government.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): Where does the Permanent Secretary serve as the senior civil servant?
MoD