Inside the new four-part documentary celebrating the enduring appeal of Dad’s Army

Attracting audiences of over 18 million in its heyday, the show still plays out on the Gold channel, and most Saturday nights on BBC2. It's been a radio series, a stage show and even a board game, and spawned two feature films in 1971 and 2016.

Among those taking part in the documentary is former Radio Times photographer Don Smith, now 86, who was on set for the majority of Dad's Army’s 80 episodes between 1968 and 1977, including the very last one in which the cast raise a toast to the real Home Guard.

Don is interviewed by Alexander Armstrong at the Royal Institution library in London and will first appear in episode two. How did he find the experience? “Alexander was very easy-going, very pleasant, as you would expect, just like he comes across,” Don told me.

“He did question me a lot, in the nicest way, about the relationships and I think I know what he was trying to get me to say. But I did say to him, when I would go along to the dress rehearsal, the last thing they would do is have disagreements among themselves, in front of me, and in front of the TV cameras and everything. All that would have been thrashed out in the rehearsal rooms or whatever."

Don began working for Radio Times in 1955, aged, 23, and officially retired in 1992, though he still visits the magazine's west-London office every week. And back in the day, Don’s policy of taking a succession of pictures, whether or not they would be used in the first suitable edition, has paid dividends.

“When I would go in and photograph a sitcom or anything, so long as the transmission was three weeks after the recording, that gave us time to photograph it and then print it. But I like to think that I always recognised what was going to be a good programme and even if it was done too late, I would still make a point of going to photograph the show, knowing that it would be repeated and that the pictures would be valuable. As is the case now.”

It’s not the first time Radio Times has come to the rescue of Dad’s Army. When Perry and Croft were compiling a book called The Lost Episodes in the late 1990s, they asked Don if he happened to have any pictures of those stories...

“I did some contact prints for pictures of the missing episodes and took them and showed Jimmy and he was absolutely astonished. He said to me, 'Why have I never seen them?' And I said to him, 'There’s more to it than that: I’ve never seen them and I took them!' I was as surprised as anyone.

Not only that, but in 2015, animators wanted to re-create one of those missing episodes, A Stripe for Frazer, but were desperate for source material. Radio Times's head of heritage Ralph Montagu located all the images taken by Don Smith during the studio recordings. "They were invaluable," producer Charles Norton told RT at the time. "Our number one research resource, in fact, because they were the only good reference we had for what the episode looked like." The animation was released in early 2016, to general acclaim.

But back to Saluting Dad's Army, and researchers have gone to great lengths to track down interviewees: among them are set designer Paul Joel; Gordon Peters, who appeared as different characters after having his scene as a fireman cut from the first episode; Caroline Dowdeswell, who played bank employee Janet King in series one; and Sir Paul Fox, the BBC1 controller who was initially resistant to the show.

In the first episode of the documentary, Sir Paul explains his misgivings about David Croft's original opening credits, which included actual war footage: "The title sequence seemed to be too brutal," he says.

There's also input from celebrity fans including actor Vicki Pepperdine ("It's a show that I hope will go on for ever"), impressionist Jon Culshaw, Outnumbered writers Andy Hamilton and Guy Jenkin, The Fast Show's Simon Day and Mock the Week captain Hugh Dennis, whose father was the vicar of the church where Frank "the Vicar" Williams was a congregant.

Saluting Dad's Army has clearly been an enjoyable job for executive producer Tom Webber, who told Radio Times, “One memory I will take away from the project is just how fondly everyone who’d been part of Dad’s Army remembered their involvement in it. Whether a member of the cast or the crew, and whether their time on the series had extended for years or just one episode, people genuinely cherished their time on the show and remembered working with David Croft and Jimmy Perry with great warmth.”

It's only fitting that Don Smith should be invited to take part, having been on set for nine years with these greats of British comedy. When working for Radio Times, Don also photographed names as big as Joan Crawford, Sammy Davis Jr, Tony Hancock, Spike Milligan, Beryl Reid, Peter O'Toole and Donald Pleasence.

In episode three of Saluting Dad's Army he recalls with affection the day Arthur Lowe greeted Don's young son before the studio recording of the 1973 story The Recruit, in which Mainwaring is laid up in hospital with ingrowing toenails.

"He got out of bed and with a greatly exaggerated walk, walked across the studio floor. He said, 'Good afternoon, young man,' took him by the hand and saluted him."

So did Don ever think when he was taking these pictures between 40 and 50 years ago that we would be talking about them now – and he would be discussing them on a documentary? "Absolutely not!"

Saluting Dad’s Army begins on Tuesday 6th November at 8pm on Gold