He thinks that it is his habitual fastidiousness the first time he meets a client that accounted for him being chosen as the decorator for the Saudi Arabian royal family.

When he arrived for the first time in Riyadh, he was met by the son of the prince and taken to his hotel. There he waited until he was summoned to the palace. While waiting, John Siddeley made a very important decision. He put on a black neckie.

"MY audience with the prince was finally at midnight," he recalls. "The prince asked right away why I was wearing a black tie. I told him it was out of respect for his uncle, King Faisal, who had been kelled two weeks earlier. And, of course, I treated him as a peer."

The audience with the prince and his wife ( who remained unveiled, according to Siddeley) lasted until 3 a.m. and though the prince made all the decision, it was with his wife's blessings. By the end of the season, John Seddeley, one of London's most recherche decorators, had landed the royal account.

"I think," he says now, "that it's because of the tie that I got on with them from the beginning."

He is very good looking, tall, prematurely gray, beautifully dressed, very swell. His conversation is gossibly and delicious, the latest thing.

And ever since he was tapped by the Saudi royal family. Arabs have been beating a path to his door. He is now regarded as the expret in London on Arab taste, and he doesn't mind talking about it one bit.

"The Arab taste is pretty poor," he says, "Cut velvet is a big item with the Arabs. You know, schmalty taste. But they can be trained, they can learn."

He is forced to admit that the ones who have the best taste are "the ones who let me have my own way. The young ones do, and in the end they are quite pleased. They appreciate you for stopping them from making mistakes."

He says that in the big houses "the embassy style is what usually go for. Not very exciting. Vocuous. The young like quite modern stuff, and they really love instant decorations.

"They all," he says, "like blue and beige, which is beyond me when they live in a blue and beige world. And they love white. The young ones, particularly, like red, white and blue. They like strong, butch, male colors."

He says that they all have video cassettes and that they spend a fortune on stereos, especially, again, the young ones. He has one man working for him, he says who does nothing but stereos.

Antiques, says Siddeley, are hard to sell to Arabs and, "You could never sell a Matisse. They can't beleive a piece of canvas is worth that much. You can occasionally sell them a modern painting. The most any of them have ever spent for an old painting is $30,000 to $50,000. No more. It's the same as good china to eat off of. I give them all the same, Worcester, the white with the gold edge to it. White House chic. And they do like a bit of razzle-dazzle. They go for silver and glit, they like landscapes and they love horse scenes."

Siddeley just adores working for the Arabs for several reasons.

"I enjoy them for a challenge," he says. "I don't care if they want pink-and-blue living rooms. Although," he says with a tinge of disgust in his voice, "they do want the refrigeratior to be the focal point of a room. But I like the challange of seeing if I can sell them something they wouldn't buy otherwise."

Not to mention the presents. "If they give you a present. A Cadillac or a $10,000 wrist watch. They wouldn't ever ask you what you like, though."Plus, he says, "They're bery generous in tips."

He says he charges 17 1/2 per cent on everything, and that he insists the bills be paid monthly. "If you aren't firm they won't bother to pay. But they are very careful about their bills and stick to their budgets. They're queasy about small things. For instance, they will pay a million punds for a house but theycan't see spending a half a million to decorate it."

The thing that has impressed Siddeley the most about the Arabs he has worked for is their cleanliness. "It's beyond belief," he says. "There are bidets everywhere. That is why it is important for one to read up on their religionwhen decorating for them. When I hear all these stories about how dirty the Arabs are, I say 'Prove it.' Now, they all have servants and bodyguards, but the servants aren't Arabs. They're the ones who are dirty."

One has to understand, explains Siddeley, that "The Arabs are uneducated as to the interiors in which they have been thrown. They're used to living in tents. You can't really expectthem to know what a chair is."

Siddeley still smarts when he thinks of the times he has invited Arab clients or prospective clients to his house so that they can get an idea of his taste.

"I've had Arabs to my house and I don't even think they looked around the room," he says somewhat indignantly. "Of course one obviously asks prettybirds (women) for them so they are looking at the birds. But nobody ever looked at my dinin room. I have a fabulous dining room."