In Restoration, published in 1989, Rose Tremain created Sir Robert Merivel, an ebullient Restoration medic. He is a sort of medical Samuel Pepys (who himself put in a couple of unnamed appearances in the novel), who tries to live the life of a rake and courtier but can't focus his energies on any one course of life. He cures one of Charles II's spaniels by leaving it be, makes the king laugh, and is rewarded by being married off to one of the royal mistresses. He was left at the end of Restoration on his estate in Norfolk having fallen in love with his wife, whom he was forbidden to touch, with a daughter by a madwoman, and with the suspicion that he might be a self-deceiving fool.

In Merivel we jump forward 15 years to 1683. Sir Robert is 56. His daughter has become a beauty, and his loyal retainers are old enough to splash the soup as they serve at table. Charles II is weary, and the nation is weary of him. Merivel himself is "weary and worn down". So he heads off to Versailles in the hope of being employed by Louis XIV. There he meets a hazel-eyed beauty called Louise de Flamanville, with whom he resumes his old rakish ways. When her husband, a homosexual Swiss guardsman, threatens to kill him ("Louise, I do believe I am about to be shot"), Merivel returns to Norfolk, along with a bear he has picked up along the way. He nurses his daughter through typhoid, operates unsuccessfully on his former mistress, who has breast cancer, and worries that the king will steal his daughter.

There is an intrinsic problem with sequels written after the passage of a long period of both real and fictional time. It can look as though ageing characters are being forced to rerun the path of old adventures at a slower rate, with more wheezing and less joie de vivre. This is certainly how Merivel seems for the first few chapters. Tremain has Merivel travel to Versailles not because he has a reason for going there but because she wants to do a bit of historical tourism. When he seems to be drooping, she makes him lead a gangbang with a whore in a carriage, even though this means he has no juice left for his Swiss mistress. Merivel too is trying to spur himself on, seeking to make something of his life by working on a treatise which proves that animals have souls. This he never finishes.

But if from some angles Merivel looks like a sequel that can't live up to the novel it succeeds, it is also clever and touching. This is because it is continually and intelligently self-conscious about being a sequel. Its characters seem always to be following patterns from the past. The most extreme example is Merivel's wife, who has gone mad and spends her days following embroidery patterns over and over again.

Merivel is himself compelled to re-enact the past in new forms: when he cuts out the cancer from his former mistress he recalls cutting his daughter from her mother's stomach. Repeatedly (and sometimes rather clunkily), he reminds us of things that he has done before in Restoration, the dog-eared manuscript of which, adorned with mouse-droppings, is discovered beneath his bed and renamed "The Wedge". As he says, when about to be forced into a second less-than-desirable marriage, "I want to say that this already happened to me long ago."

All of this means that Merivel, which begs to be described as a "merry romp", is actually a more unusual thing: a melancholy romp. Merivel says to his old mistress, "We are all dying, Violet." She, fresh from a romp with Charles II, replies "but now that I have been fucked by the king, I can die happy. Is this not so?" This note of forced gaiety is very much that of the novel. It is about late middle age as a period in which people strive to reanimate themselves by memories of what they were, but in which they end up being caught by their own characteristic compulsions and finally by their own mortality.

Merivel creates a picture not, like Restoration, of an age and of a spirited hero, but of what it's like to grow older, to see people you love die, and to witness yourself repeating old compulsions in attenuated forms while you force yourself to stay cheerful. A sequel that looks back to but does not quite recapture the spirit of an earlier novel is the perfect form in which to evoke that feeling of having to carry on, and of trying to make yourself have fun even when it starts to hurt.

Rose Tremain made her name with Restoration, the tale of physician Robert Merivel's rise and fall at the court of Charles II. Now, 23 years and six novels on, she has returned to the same subject, taking up Merivel's story in a sequel set two decades later, in the 1680s.

When he appeared in 1989, Merivel was truly the man of the Thatcherite moment, an individualistic, hedonistic creature who held up a mirror to his audience. So does he still have something to say to us in 2012? Resoundingly, yes. Merivel is a more serious novel than Restoration, and its backdrop offers a cautionary tale for austerity Britain. The suffering of its dispossessed and ill is absolute, and the absence of any support for the most vulnerable threatens anarchy and moral chaos. Merivel himself has become older and a little wiser and personal gain is no longer enough to make him content. The health and happiness of others has become central to his existence, and his sense of his failure to meet obligations to patients, tenants and mistresses looms large.

The novel opens with its hero ensconced at his Norfolk estate of Bidnold, where he has lived peacefully for many years. Life has treated him well: he is comfortable, well fed and assured of the love of his daughter and the affection of the king, who treats Bidnold as a refuge from the cares of state. But despite all this, he is melancholy, preoccupied by his failure to achieve anything very substantial. So he seeks diversion at the court of Versailles, where he meets Louise de Flamanville, a woman of voracious intellect and sexual appetite trapped in a loveless marriage. Merivel's adventures with Louise take him to Paris and Switzerland and prompt him to adopt a caged bear who wreaks havoc among the sheep farmers back at Bidnold. Along the way, Merivel's inquiring vision reveals the details that animate his world: the squalor of the backstage areas at Versailles, the horrors of 17th-century medical treatments, and the colours of rooms, paintings, coats and landscapes.

As in his earlier outing, Merivel is a rambling, shambolic figure who is well matched by a determinedly picaresque narrative. Tremain admirers hoping for the elegant, controlled plotting of her recent Trespass may be disappointed not to find similar intricacy here, where voice, rather than dramatic precision, is allowed to dominate. But although Merivel's story is sublimely untidy, it is given an emotional coherence by the relationships at its heart.

Throughout his journeying, Merivel remains devoted to his daughter and the king, even when circumstance takes them far away from him. His relationships with two other important characters from Restoration, his Quaker friend John Pearce and his servant Will Gates, are more complicated in this instalment of his story, since Pearce speaks to him from beyond the grave only and the mutual loyalty between him and Gates is tested by age and infirmity. Nevertheless, these relationships act as a focus for Merivel's unruly feelings and run like tapestry threads through a panoramic depiction of 17th-century England.

The other thing that draws the different strands together is Merivel's constant preoccupation with himself. At the beginning of the novel, he stumbles across his manuscript autobiography, here called not Restoration but "The Wedge", because for years it has been used to hold together a creaky bedstead. Reading this account of youthful adventures is both fascinating and painful for a man so immersed in his own experience, and it forces him to focus on possible endings to his story. Tormented by a sense that "The Wedge" is a poor way to be represented to posterity, Merivel tries to become a writer of treatises. But although he wants to be Montaigne, with whom he shares a desire to anatomise all aspects of the human experience, he has more in common with Pepys, the dominant model for his fascination with bodily functions and appetites. Like Pepys, he is driven to relate his worst impulses as well as his best, so one moment he is nobly rescuing his servant's body from a pauper's grave and the next availing himself of the services of a prostitute on the floor of a crowded stagecoach. And, as with Pepys, it all amounts to a rich, glowing portrait of an individual, so that what is left to Merivel and to us at the close is his writing, his "record of existence", and his corresponding sense of self, emphatically "still there in the world".

Daisy Hay is the author of Young Romantics (Bloomsbury)