Born in Forest Gate, Essex, Hardiman is often seen playing authority figures, and has played Nazi era German military personnel (Secret Army, Colditz, Wish Me Luck and Enemy at the Door) and British officers (When the Boat Comes In), police inspectors (Juliet Bravo and Softly, Softly), doctors (Home to Roost, The Royal), barristers (Crown Court and The Brittas Empire), judges (The Bill and The Courtroom), Father Abbot Radulfus (Cadfael), and of course an evil headmaster (The Demon Headmaster). He also played Prime Minister Ramsay MacDonald, in Richard Attenborough's Gandhi.
Hardiman's television work has included him playing barrister Stephen Harvestey in Granada Television's Crown Court from 1972 to 1983. Hardiman's portrayal of Major Reinhardt, in Secret Army, the (fictional) head of the Luftwaffe (German Air force) police, often provided a foil to Clifford Rose's more brutal S.S. Chief, Kessler.
As with many of the characters in Secret Army, Reinhardt was not a one dimensional villain, but a rounded character who often questioned his own motives and methods. The actor's portrayal allowed the viewer to sympathise with this character. In 1990, he played a two dimensional German officer, General Stuckler, in the final series of London Weekend Television's Wish Me Luck.
Hardiman starred as the eponymous The Demon Headmaster in the children's television drama series, which ran to three series from 1996 to 1998, and proved popular with adults as well as children.
He also had a starring role as Charles Pooter in the 1979 television adaptation of George and Weedon Grossmith's Diary of a Nobody, a role which shows his versatility more than the succession of officers he usually portrays. Hardiman's other recurring role was as Brother Cadfael's boss, Abbot Radulfus, in the television series. Joining him throughout the series as Prior Robert was Michael Culver, who had played his predecessor Major Brandt in Secret Army.
An notable small role Hardiman played was a version of the Sergeant Wilson character from Dad's Army, in an episode of time travel comedy Goodnight Sweetheart, called "Don't Get Around Much Any More", in which Nicholas Lyndhurst's character Gary Sparrow goes back in time to a bank in the 1940s, and encounters characters called Mainwaring and Wilson. Hardiman's portrayal was a keenly observed impersonation of John Le Mesurier's own performance, incorporating many of the tics and mannerisms of the original.
Hardiman also appeared as Grand Wizard Egbert Hellibore in four episodes of The Worst Witch. Hardiman appeared in the second series of The Worst Week of My Life. Another notable recent role was as a devious Swiss murder victim in an episode of crime mystery series Jonathan Creek.
He made an appearance on the daily soap on BBC One, Doctors, on 3 April 2009, as well as from 14 to 15 January 2015, and in the drama from Yorkshire, Heartbeat, as John Upton in one episode. In 2009, he appeared in a film for the BFI, Radio Mania: An Abandoned Work, directed by British artists Iain Forsyth and Jane Pollard. The film was based on The Man from M.A.R.S. (1922). Hardiman appeared as Hawthorne in the episode of Doctor Who, "The Beast Below", on 10 April 2010.Running Scared (1972) as Doctor
Pope Joan (1972) as Cardinal Anastasius
Loophole (1981) as David
The Bunker (1981) as Hermann Fegelein
Gandhi (1982) as Ramsay MacDonald
Sahara (1983) as Browne
God's Outlaw (1986) as Thomas Cromwell
Mask of Murder (1988) as Dr. Paul Crossland
Distant Shadow (2000) as The Svit
Fishtales (2007) as Professor Ratcher
Mr. Turner (2014) as Gallery Visitors