Leach was born at Much Wenlock, Shropshire. Her parents were teachers related to the social Anthropologist Edmund Leach. She attended grammar school and RADA. After appearing in repertory theatres and the Old Vic she became well known to UK TV viewers between 1965 and 1969 for playing Susan Wheldon, the mistress of building tycoon John Wilder (Patrick Wymark) in the TV boardroom drama The Power Game.
She appeared in several TV mini-series, including Germinal (1970), The Jewel in the Crown (1984), The Charmer (1987), The Buccaneers (1995) and Berkeley Square (1998), and had a recurring role on the sitcom My Family (2003-07).
Her film roles included David Essex's mother in That'll Be the Day (1973), Ghost in the Noonday Sun (1973), the TV remake of Brief Encounter (1974), S.O.S. Titanic (1979), and a voice role in the animated film of The Plague Dogs (1982).
In 1982 she played Aunt Fenny in The Jewel in the Crown and 1986 in a Jack Rosenthal British television Christmas play Day To Remember. She played a leading role as smitten Joan Plumleigh-Bruce in the six-part ITV 1987 production of The Charmer which starred Nigel Havers.
In 1987, she was nominated for BAFTA's Best Supporting Actress for A Room with a View (1985). In 1992, Leach starred in An Ungentlemanly Act, a BBC television film about the first days of the invasion of the Falkland Islands in 1982, portraying the real-life Lady Mavis Hunt, wife of the islands' then governor, Sir Rex Hunt. In 1995, Leach participated in the popular BBC miniseries The Buccaneers a five-part television adaptation of Edith Wharton's unfinished novel. Leach appeared as Selina Marable, Marchioness of Brightlingsea.
Leach played the part of Anna in BBC Radio 4's No Commitments, and Susan Harper's mother Grace in My Family. She made a guest appearance as 'Bessie' on Waterloo Road (the TV series), in Series 3 Spring Term. Since 1994, she has made occasional appearances in The Archers as Ellen Rogers, the ex-pat aunt of Nigel Pargetter. She portrayed Miss Twitterton in the Radio 4 adaptation of the Lord Peter Wimsey story Busman's Honeymoon, first broadcast in 1983.
In 2001, she played a leading role in Destroying Angel, an episode of Midsomer Murders.
She played Queen Elizabeth II three times: in the 2002 television movie Prince william; in a 2006 updated edition of The Afternoon Play entitled Tea with Betty; and in 2009's Margaret. She played "Miss Plum" in an episode of Heartbeat called "Strangers Meet on a Train" in 2004.
