PARADISE POSTPONED. By John Mortimer. Viking. 374 pp. $17.95.

WHEN, EARLY in Paradise Postponed, it is revealed that the village of Rapstone Fanner is represented in Parliament by a character named Leslie Titmuss, the concatenation of comic names suggests that we are about to enter J.B. Priestley country and that there is going to be a lot of pipe smoke, nappy ale, and high-hearted adventure in the green lanes of Merrie Englande.

Alas, in fact, there are, since Simcox's Brewery supports the family whose adventures form the core of the book, but we are early advised that Merrie Englande is as far away as the dusty works of Jeffrey Farnol. The Rapstone Valley seems a locale spared the splay-fingered touch of progress but, "After a deeper acquaintance with the place you may realize that the flint cottages have been converted to house a pop star or a couple in advertising and the roof of what looks like a farm-building now covers an indoor swimming-pool with sauna attached in which guests flop like woozy porpoises after Sunday lunch."

John Mortimer's publishers proclaim Paradise Postponed as a novel in the tradition of Trollope, but it is not the jolly Trollope of Barchester Towers we meet here, but the man who saw England in moral disarray in the dark pages of The Way We Live Now. Britain-bashing is an English sport much older than Trollope, but it is at one of its peaks in recent seasons when half the plays in London seem to be about disappointed idealists who, late in the second act, leave their homes and families for some unspecified dark destiny forecast in a speech about the futility of Town Planning, Education, the invasion of Suez, or the National Health.

Unlike some of his fellow bashers, whose indignation sometimes leads to awkward flailings, Mortimer has one Olde English virtue. Paradise Postponed is as well and tightly constructed as one of Nelson's ships. Its plot is complicated, but the development of it is clear, logical and well-paced. Moving back and forth between the end of World War II and Mrs. Thatcher's England, it makes, as it unwinds its threads, the appropriate changes in mood so that the time machine never grinds its gears.

Paradise Postponed succeeds on two levels, as puzzle story involving a peculiar legacy and as a commentary on the dizzying and sometimes depressing pace of social change. If it fails it may be in the lack of a Nicholas Nickleby, a character who encapsulates the virtues and gives us a rooting interest. Such characters, of course, are now as rare as the Ralph Nicklebys who try to bring them down, but Mortimer is just old-fashioned enough to make us miss them.

Perhaps, however, Mortimer is telling us that this new world is too big for individual heroes and villains, that Fred Simcox's unchanging survival is a marvel to be celebrated, that villains are not people any more but entities like a motel chain which tarts up Rapstone Fanner's old hotel with piped music, artificial teas and "The Old Father Thames Carvery" or a movie company which commissions Fred's brother Henry to do the script for a modern nonsectarian version of The Canterbury Tales.

IN THE Rumpole TV series Mortimer created a colorful old rascal who mingled geniality and venality as he triumphed again and again over the faceless powers of Things as They Are Becoming, but for all his Pimpernelian panache Rumpole knows as he looks into his glass of "plonk" at the wine bar that he is the last of the Mohicans and that the virtues he espouses are not simply defied and derided, but no longer even understood.

In his old age Walt Whitman spoke of himself as a battered conch on a beach, and his abounding ill health at the time makes the image understandable. Fred Simcox seems to crawl into the conch shell before he has to, but perhaps, younger than Rumpole, he is more aware of what the world he will live to see will hold. Perhaps we must think of him as being one with those Irish monks who spent a hundred or so years keeping regular hours and copying things, not with any belief that windmills can be brought down by a well-placed lance thrust, but just because somebody has to open the mail and keep the pipes clear.

Fred Simcox defies no one but keeps up a steady beat on his drums as in his life. At the end of Paradise Postponed, we may not march to Fred's beat, but at least we nod our heads to it and find hope in its steadiness.