By Tim Masters
Entertainment reporter, BBC News
Warning: this story may contain double entendres
Rather like the saying that Londoners are never more than six feet from a rat, it sometimes feels like no TV viewer is more than a couple of channel hops away from a Carry On.
Carry On films are 50 years old and showing no sign of retiring from our screens.
Whether it's Kenneth Williams with his haughtily-flaring nostrils or Sid James cackling over Barbara Windsor in the shower - love 'em or hate 'em, they are a British institution.
On Sunday, surviving stars and crew of the 31 films along with hundreds of fans will descend on Pinewood Studios to celebrate the innuendo-packed film series that started in 1958 with Carry On Sergeant.
Among the famous invitees are Leslie Phillips, producer Peter Rogers, Valerie Leon, Frank Thornton, Dora Bryan, Anita Harris, Bill Maynard, Shirley Eaton and Fenella Fielding.
Another guest will be Norman Hudis, the screenwriter of the first six films, who is flying in from California to launch his autobiography.
Then and now: Norman Hudis wrote the first six films
Now 85, Hudis wrote Sergeant when he was 34 and then went on to pen Nurse, Teacher, Constable, Regardless and Cruising, the first Carry On in colour.
Sergeant tells the story of a bunch of raw recruits who overcome their ineptitude to help their retiring sergeant win a bet.
"I realised when I started to write the autobiography 49 years later that the root of Sergeant was an incident in Egypt during the war," says Hudis.
"It was the sergeant who took charge of that situation and turned it round in an act of extraordinary daring."
Carry On Nurse, meanwhile, was inspired by stories from Hudis's wife Rita, a former nurse.
"I used to call downstairs to Rita and say 'put on the old nurse's cap and tell me something funny'," laughs Hudis.
In his book, Hudis reveals that he carried on writing Carry Ons after he left the series in the early 60s in the hope that he might be asked back one day, drafting Carry On Under the Pier If Wet and Carry On Shylock Holmes.
THE HUDIS ERA CARRY ONS
Carry On Sergeant - 1958 (pictured)
Carry On Nurse - 1959
Carry On Teacher - 1959
Carry On Constable - 1960
Carry On Regardless - 1961
Carry On Cruising - 1962
He admits that severing his relationship with the Carry Ons was a difficult time.
"I was disappointed, but it was quite clear that I was tired. I'd done six and a couple of other films and a TV series. They didn't like my script for Spying - which I was very unhappy with myself - and that was it.
"But no hard feelings - it was a glorious time."
Hudis only once met Talbot Rothwell, who took on writing duties for the next 20 Carry On films and steered the series in an increasingly bawdy direction.
'Slap and tickle'
"He was miraculous, he really churned these things out and some of the stuff was classic," says Hudis. Rothwell died in 1981.
Carry On author and historian Robert Ross describes the Carry On films one of the few major success stories of homegown cinema, putting their longevity down to the ensemble cast.
"Sid James, Kenneth Williams, Charles Hawtrey and Hattie Jacques were impeccable players and people warmed to them at an early age," he says.
CLASSIC CARRY ON QUOTES
Fenella Fielding: "Do you mind if I smoke?" (Screaming)
Kenneth Williams: "Frying tonight!" (Screaming)
Kenneth Williams: "Infamy! Infamy! They've all got it in for me!" (Cleo)
Hattie Jacques: ""I'm a simple woman with simple tastes and I want to be wooed." Kenneth Williams: "Ooh, you can be as wooed as you like with me!" (Matron)
Kenneth Williams: "She always was a weakly woman, Sire." Sid James: "You're telling me - once weekly!" (Henry)
Anita Harris: "Matron doesn't approve of banging in the ward." (Doctor)
Kenneth Williams: "You must be circumspect." Peter Butterworth: "Oh I was, sir, when I was a baby" (Don't Lose Your Head)
"The Carry Ons reflected a lot of social change, and that's what's made them so loveable. They are still fundamentally very funny films - they are all about sex and that never goes out of fashion."
So what is the quintessential Carry On film?
For a complete virgin (cue Sid James-style "yak! yak! yak!"), Ross recommends watching Carry On Matron.
"The historical films are probably better made, but in terms of slap and tickle knockabout comedy, I think the medical ones are brilliant."
Despite an unsuccessful attempt to revive the series in 1992 with Carry On Columbus, another Carry On - London - is due to start shooting this year.
Norman Hudis is open-minded about the Carry On brand continuing 50 years on.
"My feeling is that - as Hattie Jacques once said - if it's good-natured and it's funny then it should be made."
And even if he isn't still writing Carry On scripts, Hudis isn't short of jokes.
"The current gag about the 50th anniversary party at Pinewood is that the next reunion might be a seance!"