A tale of two novels inspired by Dickens - one of them a tour de force; Magwitch, by Michael Noonan. New York: St. Martin's Press. 222 pp. $11.95. The House of Cards: A Victorian Novel, by Leon Garfield. New York: St. Martin's Press. 296 pp. $12.95.

It's one of the remarkable ironies of literary fashion that the novels of Charles Dickens are little read today, except by that hard core of admirers who prize him (along, alone perhaps, with Shakespeare) as a great writer who was also a superb popular entertainer.

Dickens's rambling plots, overuse of coincidence, and unrestrained sentimentality offend readers who believe novels should be trim and well-mannered; the calculated breadth and detail of his books strike many as mere wretched excess.

I wonder, though, if readers a hundred years or so from today may not find the sexual forthrightness and cynicism of today's fiction equally awkward and naive; stale conventions and contrivances that more sophisticated readers have outgrown.

All this self-indulgent speculation is prologue to a consideration of two recent novels in the Dickensian manner by two British writers.

Michael Noonan's ''Magwitch'' offers an expansion of the core material of Dickens's masterpiece, ''Great Expectations'' (1861), whose readers will recall how its hero, Philip Pirrip (Pip), was raised to the status of a gentleman through the efforts of an escaped convict (one Abel Magwitch) whom he had befriended, then returned to reality when his benefactor's ill-gotten fortune was confiscated by the crown.

In Noonan's continuation of the story, Philip, now a grown man employed by a successful shipping firm, takes advantage of a business trip to New South Wales to investigate how Magwitch prospered there, to follow up clues that suggest much of his wealth remains hidden there, and to encounter - to his mingled delight and confusion - a beautiful half-breed girl who he knows must be the half-sister of the girl he loved and (we now learn) lost, the imperious Estella.

It's an intriguing idea that doesn't work, because Noonan inexplicably limits the book's scope to the raising of possibilities and questions and their slow resolutions. All is summary and revealed information; there's little drama beyond the obvious conflicts of rival fortune seekers and almost no effort at characterization - even though the unforgettable Mr. Jaggers of ''Great Expectations'' turns up and manages to kindle the book into brief life. What's missing in ''Magwitch'' is any sense of the complex wrestling with issues of ambition and responsibility that gave its great original its masterly pathos and depth.

Leon Garfield's ''The House of Cards,'' by contrast, seems to me a complete success. Garfield is a renowned writer of children's books whose claim on adult readers was demonstrated by his recent (1981) ''completion'' of Dickens's unfinished final novel, ''The Mystery of Edwin Drood'' (1870).

The imagination that made that earlier book a gem among Dickens imitations is again on display in this full-scale neo-Victorian novel, a complicated tale of detection, romance, anarchy, and mistaken identity that steadily, wittily engrosses us by means of its rich characterizations, vigorous language, and skillfully manipulated plot.

The story begins in 1847 in a massacred Polish village whose one survivor, an orphaned baby, is rescued, grudgingly, by a misanthropic ''tramp.'' Flash forward to a dozen years later: London - the shop where ''Famous Pickled Herrings and Continental Delicacies'' are purveyed by the energetic family of Mr. Dolly, a Polish Jew (nee ''Dolska'') whose habit it is to collect impecunious friends and look for unassuming ways to increase their fortunes without making them feel beneficiaries of charity.

At one of Mr. Dolly's famed Friday night dinners, the company are interrupted by the sudden appearance of Katerina Kropotka, a Russian lady of disturbed demeanor and mysterious origins. Attempts to aid her precipitate a bizarre sequence of events that reaches backward to that destroyed village, and forward to include a complicated lawsuit, a murder, and several intrigues which develop from, and cleverly echo, these dramatic major events.

Garfield fills this concise yet crowded story with fascinating characters; like Dickens, he's best with minor personages such as the persnickety Irish maid Brandyella and the self-effacing Mrs. Fairhazel (determined to ''keep smiling even if she should be eaten by lions'') - though he succeeds best with the diligent lawyer's clerk, Mr. Clarky, a servant of the law, whose rectitude is movingly portrayed.

The spirit of Dickens also breathes through the resourceful metaphoric language with which Garfield brings every scene vividly alive. (See, for example , the wonderful suggestiveness he gets from clusters of hanging sausages in Mr. Dolly's shop, or the symbolic power in his description of the prison door, which conceals the last of the mysteries that Mr. Clarky deciphers.)

Even readers who think they don't like Dickens should have a go at ''The House of Cards.'' It's one of the most satisfying novels I've read in recent years.