It was while playing Henry V that Hardy developed an interest in medieval warfare, and he later wrote and presented an acclaimed television documentary on the subject of the Battle of Agincourt. He was one of the experts consulted by the archaeologists responsible for raising the Mary Rose.
Over the years, Hardy has played a huge range of parts on television and film, but has never become widely known outside the UK. One of his most successful parts was that of Siegfried Farnon in the long-running All Creatures Great and Small (1978). He also played Robert Dudley, Earl of Leicester in Elizabeth R and Prince Albert in Edward the Seventh. He took a leading role in the 1995 film version of Sense and Sensibility.
He is the son-in-law of the late Gladys Cooper.