Aged 20, Hazeldine made his London debut at the Royal Court Theatre in Sloane Square. Initially hired to understudy as Cliff in Look Back In Anger, Hazeldine soon started taking on small roles in Edward Bond's Narrow Road to the Deep North and Early Morning. He worked constantly at the Royal Court during the 1969-70 season. Spotting Hazeldine's potential, Peter Gill then cast him in his play Over Gardens Out and went on to direct him in Crete and Sergeant Pepper by John Antrobus.
His film credits are less numerous. Hazeldine made his screen debut as Stalin in Nicholas and Alexandra (1971). His credits also include The National Health (1973), The Medusa Touch (1978) and Pink Floyd – The Wall (1982).
Other major TV roles include that of Frank Barraclough in the 1973-75 drama series Sam and the role of Journalist Tom Crane in the 1979 paranormal thriller series The Omega Factor. He also starred in the children's series Chocky (1984) and its sequels Chocky's Children (1985) and Chocky's Challenge (1986), all scripted by one of The Omega Factor's Writers, Anthony Read. In 1983 he played the part of Kidder in Willy Russell's five part Channel 4 drama One Summer, also starring David Morrissey. In the later 1980s Hazeldine had major roles in two BBC comedy dramas: as haulage firm owner Picard in Truckers (1987) and as Bernie in Streets Apart (1988–89). In 1999, he played Austin Danforth in the ITV drama The Last Train and in 2001 he appeared as Ivan Braithwaite in Adrian Mole: the Cappuccino Years.
Although Hazeldine was rarely cast in starring roles, he became noted as an exceptionally honest and truthful actor who after his inductory season at the Royal Court was regularly seen in new plays, e.g. The Old Ones by Arnold Wesker, The Foursome by EA Whitehead and Cato Street by Peter Gill. In 1981 he joined the Royal Shakespeare Company for a season to appear in productions of Troilus and Cressida, Timon of Athens, Richard III and Edward Bond's play The Fool.
In 1984 Hazeldine made his Broadway debut as Sam Evans in Strange Interlude with Glenda Jackson. He returned to New York in 1998 when he played Harry Hope in The Iceman Cometh starring Kevin Spacey. His last stage performance was that of the guilt-ridden Joe Keller in Arthur Miller's All My Sons (2000), praised by John Peter of the Sunday Times as "the subtlest and most harrowing performance of his career."
Hazeldine was also a prolific television actor who is probably best known for his role as firefighter Mike "Bayleaf" Wilson in the LWT drama London's Burning. He played the role from 1986 to 1996 and also directed some episodes.
At the time of his death Hazeldine was appearing at the Cottesloe Theatre as Sigmund Freud in Christopher Hampton's play "The Talking Cure". He was taken ill on 10 December 2002, the day after having performed in a preview of the play.
