By 1962, the once innovative Dixon of Dock Green (BBC, 1955-76) had settled into a cosy predictability, its avuncular hero inhabiting a world of low-level criminality, of gentlemen thieves and wayward youths, easily solved crimes to be followed by a nice cup of tea and Dixon's reassuring homily.

Writer Troy Kennedy Martin, for one, was convinced that the police drama badly needed an injection of energy and bite. The result was Z Cars (BBC, 1962-78). Set in the fictional Northern overspill town of Newtown and its neighbour Seaport, Z Cars took the genre into new territory, with a variety of more complex characters in place of a single spotless hero - the first episode portrayed one officer as a compulsive gambler, another as a wifebeater - and an assortment of lowlife villains - muggers, small-time gangsters, corrupt landlords, pimps - of the kind that would become a staple of TV cop shows. It was deliberately issue-driven, with storylines highlighting problems of poverty, racism, mental illness and addiction.

Although the series was, in the convention of the time, mostly shot live in the studio, it featured an unusually high proportion of filmed inserts, lending it a more convincing appearance of 'reality' which hinted at the increasing incorporation of documentary elements into TV drama as the 1960s wore on.

To the surprise of the BBC, the show was an instant hit, with audiences rising to 14 million before the end of its scheduled 13-week run, which was hastily extended to 31 episodes. But fears that its popularity would stifle creativity and bring stagnation led Kennedy Martin and fellow writer John McGrath to move on. Nevertheless, thanks to the contribution of other writers, notably John Hopkins and Allan Prior, and directors like Ken Loach, Z Cars continued to provide imaginative and challenging drama, while characters like the acerbic DCI Barlow (Stratford Johns) and the hotheaded PC Fancy Smith (Brian Blessed) became audience favourites without losing their rough edges.