It's just one of the rules he sticks to. The modern version of the sitcom might be rather good for all he knows but the creator of Last Of The Summer Wine and Keeping Up Appearances - among many other successes - refuses to watch anything he hasn't written himself because he's worried he might unconsciously steal some of the ideas. The funny thing is, though, in a way, stealing an idea is exactly what he's done with his new comedy. It's just that he has stolen it from himself.
The idea - which he first had in the early 1970s - was for a sitcom about a shop, and the two men who work there. After a pilot in 1973, the idea eventually became Open All Hours starring Ronnie Barker and David Jason, and ran for 26 episodes. Now, almost 30 years on from the last episode in 1985, Clarke has revived the idea and, although it's just a one-off episode at the moment (to be shown on Boxing Day), with a bit of luck, he says, it might become a series. The old corner shop will be open for comedy business again.
Many elements of the new episode, which is called Still Open All Hours, are the same as the original series; some are different. Arkwright, the owner of the store, is gone of course (Barker, who played him, died in 2005) but many of the other ingredients are still there, including Nurse Gladys Emmanuel, the bosomy midwife and the object of Arkwright's desires, who is once again played by Lynda Baron. The series' greatest comedy invention is also still there: the temperamental till that would snap at Arkwright's fingers when they weren't being slapped away by Nurse Emmanuel.
And Granville is back of course and is still played by David Jason. In the original series, he was a stamped-on, sex-starved young man and the nephew of the stuttering, sex-starved Arkwright, and in almost every episode he would dream of an alternative life to a corner shop. "If I wasn't here right now cleaning down this bacon slicer," he said wistfully in one episode, "I'd like to be flying a jumbo jet into Kennedy Airport."
In the new series, he is about three decades older and none of the dreams have come true, although one of Arkwright's predictions certainly has. "Granville," he used to say, "you are my nephew and one day all of this will be yours." And that is exactly what has happened: Granville is now running the shop, pretty much along the same lines as Arkwright.
He stands defiant against all the changes there have been to shopping ("You don't get the personal touch at the supermarkets," he says). He avoids spending money whenever he can ("You're a tightwad, like your uncle," says Nurse Emmanuel). And he uses every trick he can to shift stock he's picked up cheap somewhere (in the new episode, it's a job lot of anchovy paste).
"I was trained by the m-m-m-master," he says looking up at a picture of Arkwright. "He was a b-b-b-black belt in being sneaky."
When I call Roy Clarke at his home in Yorkshire to talk about the new episode, he tells me that he had a lot of fun returning to Granville and taking up his story.
"The bottom line is that he's become Arkwright, more or less - he's just as stingy. He was trapped, but he makes the best of it and has probably done pretty well out of the business. There were probably some rented properties going that Arkwright had never told anybody about so he's been fairly comfy even though he has to work all those hours. It's all he's ever known and he seems content enough."
As for the shop itself, it has barely changed, and Clarke insists there are still shops like that in Yorkshire (the shop in Doncaster where the series is filmed is now a hairdresser).
Arkwright's shop itself was originally inspired by the one Clarke's wife ran when they were first married. Clarke was working as a teacher at the time (he also worked as a policeman) while his wife Enid ran the corner shop, and he could see how hard it was. "Running a shop was purgatory," he says, "Like publicans, a shopkeeper is never off duty."
Clarke remembers that shop his wife ran well and the kind of customers who came in: redoubtable women who would take no nonsense and were out to get a bargain at any cost.
"Those women are the great strength of this area," says Clarke. "I used to see it in the mining communities when I was a policeman. When those guys came up from that awful hole in the ground, they used to enjoy themselves and, without some woman at home keeping things on a level, they'd have been in chaos. There are some really strong women round here and I recognise some of the women in my life."
Clarke, who is now 83 years old, has put those women at the centre of Still Open All Hours, making it in many ways a celebration of the matriarchal society (the men think they are in charge, but the woman are really).
Clarke also says he has enjoyed revisiting that idea, with the likes of Stephanie Cole and Lynda Baron keeping Jason in his place, but admits it hadn't actually occurred to him to revive Open All Hours until the BBC approached him and suggested it. After he agreed - with pleasure - to do it, he met with Jason and they talked through the character, where he might be in his life and how the show could work.
The trick, says Clarke, was to try to recreate the atmosphere of the original series and remind audiences why it was so loved. Clarke isn't entirely sure himself why that was, but does have one theory. "It was a kind of music hall theatre," he says, "because you have this stage and these acts come in and do their bit and then go again - it was almost like a piece of variety."
In the new episode, Clarke recreates that variety-show atmosphere with a series of short starring roles for well-known comedy performers who pop into the shop and then pop off again. Stephanie Cole is particularly good as the septuagenarian vixen Delphine Featherstone, Barry Elliott (one of the Chuckle Brothers) provides the funniest moment of the episode when he briefly appears at the door of the shop trying to control an unruly dog, and Johnny Vegas pops up as another sex-starved man who puts the anchovy paste to good use (in the wrong place).
It is interesting that Vegas's character should be sex-starved because, although Open All Hours is on the face of it about shopping, isn't it really about sex?
Arkwright was always desperate for it, as was Granville (although there's a suggestion in the new series that he finally got it).
What makes Open All Hours unusual now is that Clarke deals with the subject of sex using the old-fashioned technique of double entendre. "That's my generation," he says, "I prefer not to deal with it directly. Let's put it this way: I'm not experienced enough to deal with it directly."
Clarke also simply prefers his comedy old-school. His influences are the likes of Buster Keaton, Will Hay and George Formby, and in Still Open All Hours there are a couple of good examples of Keatonish slapstick. Clarke also likes to keep things warm and friendly too; there is no nastiness in his comedy; there are no sharp edges.
"There's much of a negative edge to modern comedy," he says. "And some people like that, but when you get older, you don't want that. You want something with a bit of warmth. Look at the warmth of people like Morecambe and Wise and Tommy Cooper - where's it gone?"
Still Open All Hours is on BBC One on Boxing Day at 7.45pm