Ian Cullen used to be in Z Cars. You'd recognise his face straight away. He was the one who played DC Skinner, the grumpy Geordie who was always having trouble with his girlfriends or his colleagues. It's almost 40 years since Cullen last played the part but, as the lead roles in hit series tend to do, the character has clung to his back ever since and refused to budge. Just the other day, when he was in Australia visiting his daughter, he walked into a bar and someone started whistling the Z Cars theme tune. That happens to him a lot, he says, but he doesn't mind. He loves it really.
The thing is though: don't let any of that fool you into thinking the story of Z Cars is just a story of nostalgia, the story of an old whistled theme tune from long ago. Z Cars may have been made in the 1960s and 1970s, but the police drama is still relevant now. In fact, if the modern TV cop show ever gets curious about where it came from and goes off in search of its ancestors, it will discover that every modern police show, here and in America - The Sweeney, The Bill, Cagney And Lacey, The Wire - has one ancestor in common. They all have the same granddaddy: Z Cars.
The evidence for this comes from Steven Bochco, the man who created one of the most influential police series of all time: Hill Street Blues. When it was first shown in 1981, Hill Street Blues was credited with reinventing the genre, of making it real and relevant, but when Bochco was visiting the British Film Institute recently, he spoke for the first time about just how much his show owed to Z Cars. Dick Fiddy, the BFI's television expert, was present when Bochco revealed this surprising connection between the two shows and says that, for anyone interested in television, it was a jawdropping moment.
"Someone asked Steve when he was setting up Hill Street Blues, had he ever seen Z Cars, and he said not only had he seen it, he made his writers watch it," says Fiddy. "And that's interesting because if Z Cars inspired Hill Street Blues, then Hill Street Blues has almost certainly inspired almost every cop show on American television since, all the way through to The Wire. Steve Bochco said: 'I wanted my writers to watch Z Cars and I wanted them to get that idea of overlapping dialogue, to get the home life in there. The BBC had already done it and I wanted to emulate it.'"
You can see what Bochco means when you look at the old episodes of Z Cars, some of which are about to be released on DVD for the first time. The core of each episode is a crime of some sort, but the focus is on the working life of the coppers and the home life of the criminals and their victims. Some of the production techniques are dated now, but the intention is utterly modern: to reflect the lives and concerns of the people watching and to entertain and occasionally shock them as well.
It's worth remembering how groundbreaking that was in the early 1960s. Z Cars, which ran for 667 episodes from 1962 to 1978, wasn't just another cop show, it was part of a radical modernisation at the BBC in response to the birth of ITV. In its first years, the commercial channel had blown the Beeb away. Then, in 1960, Hugh Carleton Greene took over as director-general, realised something had to be done, and within a few years the BBC was making That Was The Week That Was, Doctor Who, Top Of The Pops and Z Cars - shows that changed the landscape and wrenched the ratings back from ITV.
Ian Cullen was aware of all of this when he joined Z Cars in the 1960s and realised he was about to be part of an important show which had already made stars of Brian Blessed as "Fancy" Smith, Frank Windsor as Detective Sergeant Watt and Stratford Jones as Barlow. "We knew it was popular," he says, "immensely so. In fact, it's possible to forget how big Z Cars was - anything that was a hit on either commercial television or BBC One used to get 20 million viewers. It was huge. But I also knew that, socially, it was important. It was saying something relevant to the time, which was quite unusual in television in those days."
Cullen, who came to Z Cars from Emergency Ward 10 and would later appear in When The Boat Comes In, believes it was this effort to be relevant and reflect the audience that made the show so successful. "Shakespeare said you must hold a mirror up to nature and that's what Z Cars did," he says. "It showed people life as they recognised it. But at the same time, it was carefully produced and written. The standards of the scripts were terrifically high, and it all depends on the quality of the script."
Cullen wrote one of the scripts himself (an episode called Missing) and he knew the trick was to write about an issue - in his case children being left home alone - without making it dull or preachy.
"You've got to be very careful," he says. "The expression is, don't sell your soul for a potted message … which some writers can do. They're so anxious to promote an issue that they don't write a good script."
Cullen was central to some of Z Car's most memorable scripts over the years. In one of the episodes about to be released on DVD, he struggles with a move from the uniform branch to CID, and it feels real. "What I liked about Skinner," says Cullen, "was he was slightly unlikeable - he was a bit of a misogynist and he was a sulker and he wouldn't take orders and didn't get on very well with his mates. He was a weird fella, but that's what made him real."
By the time Cullen left the series in 1975 - killed in a shootout - police drama had changed again. ITV was producing The Sweeney, with John Thaw as Jack Regan, and that had started to make Z Cars look cosy in the way that Z Cars had done with Dixon Of Dock Green 10 years before.
Cullen, however, believes it wasn't The Sweeney that killed off Z Cars, it was the constant pressure the BBC felt to do something new. "It wasn't dated," he says. "It was just that the BBC had a thing about a responsibility to be doing new things and keeping something on for 16 years was criticised."
Looking back now, Cullen still rates Z Cars as one of the best police dramas ever and recognises its importance in influencing the shows that came after it. Dicky Fiddy of the BFI goes even further and says you can switch on your television right now and see the influence of Z Cars in contemporary cop shows such as New Tricks and Luther. He thinks there's a subtly different factor at work there: a kind of nostalgia for the policeman portrayed in Z Cars.
"We're used to the idea of the cynical cop now, the flawed policeman, whereas we weren't at the time of Z Cars, certainly not on the BBC," Fiddy explains. "What a show like Life On Mars proved is that there's a nostalgia for that type of cop show - it's the reason why Life On Mars and New Tricks are popular.
''One of them is about a modern-day cop dealing with the vagaries of the past, the other is about old cops being brought back into the present day. It's nostalgia not for an old style of police but an old style of police show. It's part of Luther's success as well - he's like a Regan or Barlow, he breaks the rules. People miss that."
Cullen agrees with that and thinks the time is right for revisiting Z Cars. He's delighted there's a DVD being released - and he loves it when he wanders into a bar on the other side of the world and someone whistles the theme tune - but he'd love to see the show being repeated, perhaps on a cable channel.
It would be nostalgia, of course, a looking-back, a remembering; but it would be a tribute too, a recognition that television may have changed dramatically in the past 40 years but it still owes a lot to one of the pioneers, the one that did it first, the granddaddy.
Z Cars: Collection One, featuring six colour episodes from 1972, is released on DVD on Monday.