Leach became a household name when in 1984 he was asked to become one of the stars in The Bill, a highly successful ITV series about the Metropolitan Police in which he played Sergeant Tom Penny.
Among these, he played Lambert le Roux in Pravda to critical acclaim, Rev Hale in Arthur Miller's The Crucible, Ebenezer Scrooge in Charles Dickens' A Christmas Carol, Bluntschi in Arms and the Man, Subtle in The Alchemist, Claudio in William Shakespeare's Measure for Measure, and Colin in Ashes, for which feat of achievement he was cast in The Bill.
Leach appeared in a West End production of Anthony Shaffer's play The Case of the Oily Levantine directed by Patrick Dromgoole, and at the Royal Court in Gimme Shelter.
In later years he featured as Uncle Max in the Sound of Music, Captain Brackett in South Pacific, the Narrator in The Rocky Horror Show, Doolittle in My Fair Lady, and in 2001, Peachum in a tour of The Threepenny Opera.
In October 2001, he read the Letters of Beethoven with the Sarum Chamber Orchestra and started rehearsals for The Donkeys Years at the Salisbury Playhouse, which was due to open in January 2002.