Dad’s Army fans have more than one reason to celebrate this week. On Friday 5 February a brand-new film version of the classic sitcom will be released in UK cinemas. But the BBC has stolen a march on the movie by releasing a lost Dad's Army episode the day before.

Although A Stripe for Frazer has not been seen since it first aired on 29 March 1969, the 30-minute black-and-white episode is being made available in animated form for new digital service BBC Store.

Three episodes from the second series have been missing for decades, as it was standard practice for the BBC to record over or discard tapes of programmes from this era. But an audio recording that came to light in 2008 means a “flash animation” of one story – in which Private Frazer is made a lance corporal – could be created.

The soundtrack was supplied by BBC West Midlands DJ Ed Doolan, who recorded thousands of programmes from his television set in the 1960s and 70s.

Animation producer Charles Norton takes up the story: “Ed has this massive library of often unique recordings of programmes that have otherwise become lost from the BBC archives. And for various complicated reasons, it’s now in my shed! There are boxes and boxes of reels of tape.”

The Dad’s Army tape, however, has found another home, requiring the talents of audio restorer Mark Ayres, who transferred it to digital files.

“The main problem was that it was recorded onto a section of tape from which the previous recording had not been fully erased,” explains Ayres, who used to write incidental music for Doctor Who in the 1980s.

“This had to be cleaned up and removed using spectral editing techniques, which enable me to literally erase the unwanted bits between the wanted – a bit like using PhotoShop to remove an unwanted photobomber!”

“To me it seemed like a really obvious idea to animate it,” adds Norton. Similar treatments have been applied to missing Doctor Who stories from the 1960s, to general acclaim among fans. But Norton admits, “I’ve been battling to get this project off the ground for about six years.” Originally this was a BBC2 initiative but a change of controller saw the project vetoed.  BBC Store later took it over.

But the animators desperately needed source material, which is where Radio Times comes in. RT’s head of heritage Ralph Montagu located nine publicity images that were taken by photographer Don Smith during the studio recordings of A Stripe for Frazer. A tenth image from the episode was actually printed in the magazine.

“They were invaluable,” adds Norton. “Our number one research resource, in fact, because they were the only good reference we had for what the episode looked like.” 

It’s not the first time that Radio Times has given Dad’s Army a leg-up. Before the first episode aired on 31 July 1968, when BBC executives themselves were nervous about the show, a far-seeing preview in the magazine said, “although Dad’s Army is set firmly in wartime – the fun itself is timeless”.

Twelve people around the country have worked flat out on drawings, backgrounds, shadings and computer renderings. “The animators have been working extraordinarily hard,” says Norton. “It is quite staggering what they have managed to achieve." 

Norton is pleased that the story about Frazer and Jones vying for Captain Mainwaring’s approval is such a strong and character-focused episode. “There are Dad’s Army episodes in which all sorts of bizarre things happen – people getting strapped to windmills or going off in barrage balloons – that would be very time-consuming from an animation perspective!

However, as Norton points out, a lot of work went into getting the characters right. “Frazer is a gift, really. He’s got such a wonderful face. It’s almost like he’s stepped out of a Warner Brothers cartoon. He oscillates between gently simmering and ‘I’m about to kill you’. He’s angry all the time.

“Some of the other characters were tricky to get a hold of. With Sergeant Wilson, you learn after a while that less is more. It was an incredibly nuanced performance. In terms of animating the character you need to leave him space to gaze off into the distance and go off into a world of his own. You don’t want to make him too busy!”

Tony Pritchard of the Dad’s Army Appreciation Society describes the project as a “wonderful initiative”, and says, “With the aid of the audio recording by Ed Doolan and the animators, a long-lost episode will be brought back to life.”

As part of the celebrations BBC Store is inviting fans to nominate their favourite episodes for inclusion in a Best Of collection by tweeting suggestions @BBCStore #BestDadsArmy or posting them on the BBC Store Facebook page.

Although audio has yet to be found for the two still-missing Dad’s Army episodes, it is hoped that animation could be used as a solution for returning to the public domain lost stories of other classic series including Hancock’s Half Hour.

Dad's Army: A Stripe for Frazer is available from BBC Store from Thursday 4 February