He reached the rank of "Assistant Secretary", (used to be Grade 5), which is very senior, (not someone who makes tea for the typist). See
https://www.civilservant.org.uk/informa ... roles.html
Sir Humphrey explained it in "Yes, Minister"...
"Well briefly, sir, I am the Permanent Under Secretary of State, known as the Permanent Secretary. Woolley here is your Principal Private Secretary. I too have a Principal Private Secretary and he is the Principal Private Secretary to the Permanent Secretary. Directly responsible to me are ten Deputy Secretaries, 87 Under Secretaries and 219 Assistant Secretaries. Directly responsible to the Principal Private Secretaries are plain Private Secretaries, and the Prime Minister will be appointing two Parliamentary Under-Secretaries and you will be appointing your own Parliamentary Private Secretary."
In the context of my ex-department, the Head of the "Laboratory of the Government Chemist" ("The Government Chemist") was a Band 3, his Deputy was a Band 5. They had nice offices, and didn't do any chemistry, but might have done in their youth.