Complex sentence: He spent the next 20 years in the Private Secretary's Office, becoming Deputy in 1986, and Private Secretary in 1990 to have earned relatively little, although this was offset by his grace and favour housing and other perks.
Split into simpler sentences: He spent the next twenty years in the Private Secretary's Office, becoming Deputy in 1986, and Private Secretary in 1990. During his tenure, he is believed to have earned relatively little, although this was offset by his grace and favour housing and other perks.