Good old Jerry loves his sleep, but one long night he gets more than he bargained for. A nasty business, this; "unpleasantness," as the chief inspector from Scotland Yard refers to undertakings like MURDER.

In "The Seven Dials Mystery," from 8 to 10:30 tonight on Channel 5, the bodies are discovered with the swiftness and finesse one expects of Agatha Christie; this is the first Christie TV adaptation to be seen in the United States (a production of London Weekend Television). Occasionally it becomes so playful that it gets silly, but by and large "Seven Dials" is capital fun and absorbing down to the last devilish turn of events.

According to host Peter Ustinov, who introduced the mystery, it is set in a time, the early '20s, "when loyalty and patriotism and truth were words of true meanings . . . and the English breakfast saw its finest hour." Probably it is one of the worlds that exist almost entirely in nostalgic imagination -- the kind of world in which a gentleman can say to his gentleman's gentleman, "I say, Stevens, pop out and get me some cigarettes, will you?"

Or -- more to the point -- "I say, Stevens, would you go out and buy me a pistol?"

The heroine of the story is an endearingly same girl, Lady Eileen Brent, whom most people call "bundle," played with plucky spunk by Cheryl Campbell. "I'm not going to let the grass grow under my feet," she says convincingly as the mystery deepends, and when asked if the last words of a dying man can be trusted, she testified, "Death concentrates the mind most wonderfully."

For plenty of crisp, wry dialogue, we can thank Christie and the adapter, Pat Sandys. Director Tony Wharmby, also the executive producer, spins the tale along like a top, and the cast also includes, to its great benefit, John Gielgud as Bundle's dottery father, and the formidable Harry Andrews as Superintendent Battle, who wisely observes. "Gentlemen who have no sense of humor tend to take themselves too seriously, and tha leads to mischief."

We are also treated to the sight of none other than Rula Lenska as a Hungarian countess. Yes, it is the real Rula.

The interiors for this production were shot on tape, the exteriors, most of them around beautiful Sussex estates, on film; but few viewers will be able to tell tape from film. The British have advanced the art of tape editing and lighting further than American television has. As a result the whole show is terrifically handsome; it sparkles visually as well as verbally.

When the villian is finally unmasked, he or she can be heard to scowl, "I did it for pleasure. Something to make the blood tingle. . . . What I really loved, really enjoyed, was manipulating everybody." It could be dear old Dame Agatha speaking herself.

If there are plenty of hearty chuckles in the program, there are also a few in the commercials, late flashes from the Mobil Information Center." A mock news anchor declares a "new spirit of determination" in the American economy and "a new dedication to economic revival promising a brighter future." A "reporter" in the field says that wonderful godsends like the "removal of regulations that impede economic growth" are making this yreaganesque Utopia possible.

For bringing American television such sweet and snappy treats as "The Seven Dials Mystery," Mobil is entitled to peddle whatever tommyrot it likes. In the past Mobil commercials have been controversial. These are just hilarious -- like the Shirly Temple movies that declared an end to the Depression three or four years early.