A vanishing breed of British gentry

Chin up, fellow Anglophiles, ''Masterpiece Theater'' is returning to a kind of ''Upstairs, Downstairs'' frivolity.

''Love in a Cold Climate'' (on PBS starting at 9 p.m. Sunday and continuing for seven succeeding Sundays, check local listings for premiere and repeats) is a deliciously superficial eight-part tea-and-crumpets soap opera. It is filled with charmingly ''dotty'' and ''ditsy'' people who engage in generally ''gaga'' relationships.

Based on two of Nancy Mitford's satirical autobiographical novels, ''The Pursuit of Love'' and ''Love in a Cold Climate,'' the series chronicles the sometimes naughty, sometimes rude (to use the British terms) activities of her own thinly disguised Oxfordshire family through the 1930s and '40s. You know you are in for a feast of eccentricity when the first segment opens with the comic tyrant of a Papa on horseback in the midst of his bloodhounds, using his children as hares and foxes in a thoroughly enjoyed-by-all ''child-hunt.''

From there on, the show proceeds to catch the Mitford-Radlett clan, consisting conveniently of four daughters and cousin Fanny in this TV version of reality, as they develop from precocious pre-adolescence into precocious maturity. For a while you will believe you are viewing a kind of upper-class comic strip, a ''Little Women Go British.'' But then Miss Mitford turns serious. The trifle turns into a deadly serious tale about the varied fortunes of the girls in a society where their main concern is to find a proper husband - ''one of our own kind.'' Amid all the constant twittering, there emerges a major theme: ''Am I in love or do I merely love him?'' There follows in the various episodes a variety of love affairs and odd relationships broad enough to provide ''The Young and the Restless'' with a year's worth of plots.

By the time the eighth episode ends on a tragic note, some viewers may be bored with the empty-headed snobbishness and pseudosophistication of much of the story, and many viewers may be offended by the casual sexuality of many of the characters. But all viewers will, at least, have gotten a fairly accurate account of the daily trials and tribulations of a whole social class in transition.

''Love in a Cold Climate,'' adapted by Simon Raven and acted by all with lighthearted skill, seems on the surface to be shallow fun and trivial drama. But in its own very British way, it is a gentle eulogy for a vanishing breed of gentry.