But from 1955 until 1976, his was the benign face of British policing. So was Dixon telling the truth? And how did the supposedly more realistic Z Cars alter our attitudes to law enforcement? Sadly, any attempt to compare and contrast these important social documents is impossible. Because while each programme ran to hundreds of episodes, only a handful survive.

The rest have simply vanished, along with much of Dad’s Army, Doctor Who, Hancock’s Half-Hour, The Likely Lads, Till Death Us Do Part, The Avengers and The Andromeda Strain.

In the early days of television, programmes were broadcast live. Where recordings do exist, they are on film, and were made by pointing a cine camera at a screen. In 1958, the BBC began recording on videotape. This new medium was expensive, but could at least be “wiped” and used again. And this is often what happened, at both the BBC and ITV, partly to save money, but also because popular culture was deemed of only passing interest (just four episodes of Top Of The Pops survive from the 1960s).

However, some programmes were thought worth keeping, and, together with those earmarked for export, these were transferred to film. But even that safeguard proved short-lived. Film cans spilled into the corridors of Television Centre, until one day in 1972 the fire brigade dropped by for a safety check. Many regard the purge that followed as an act of vandalism.

Recent appeals to viewers who might have taped early shows have turned up some treasures, and many films that were sold abroad have also been recovered. But much that survived well into the 1970s has clearly been lost forever. Today, the BBC is obliged to throw nothing away without first offering it to an independent archive. And with all material now being converted to digital format, caution is at last the watchword - or, as Dixon would have said, “Mind how you go”.

DAVID NEWNHAM