There’s something irresistible if slightly solipsistic about researching your own ancestry. Thanks to digitised and searchable records, all the frustrating blanks in your family tree can now be enticingly fleshed out. Sometimes those blanks will reveal something remarkable and dramatic, but as any fan of BBC One’s Who Do You Think You Are? will attest, even the mundane tales can be strangely touching.

Simon Mawer, Booker-shortlisted for The Glass Room in 2009, now enters this arena with his new novel (his word). Composed largely of fictionally presented chapters from the lives of his own 19th-century forebears, its narrative progress is peppered with authorial interruption as Mawer seeks to remind us of its underpinning reality. These interruptions come in many forms: photographed register entries in copperplate; his own musings on the nature of his project (“It could be the start of a Dickensian novel, couldn’t it?”); and somewhat pedantic footnotes making sure we know that those are the “very words, taken from his letter home” or that he isn’t misspelling “Babarbadoes”.

From that “Dickensian” opening on a beach in Suffolk, where young Abraham Block strips a drowned corpse of its two gold sovereigns, we move into his adult life at sea. But with procreation as the inevitable driver of the story, we soon swipe across to a train carriage with an ingenue seamstress coming up to London for the first time. And there’s a bounder in the seat next to her, exploiting their forced proximity to initiate his seduction. Still, she lands on her feet when, now with child, she rents a room with Abraham’s uncle near the docks. And thus Mawer ancestry, on the maternal side, is under way.

Part two, of course, requires a complete restart, with a shift over to the paternal side of things. Thus we dive into the life of George Mawer, private soldier in the 50th Regiment of Foot. Beginning with his marriage to one Ann Scanlon, we move with him from barracks to garrison and back again. She, like all army wives, will share his curtained-off dormitory bed, and offspring soon follow. But family life is curtailed by the British government’s decision (maybe not for the last time) to do something about Russians in the Crimea. George’s regiment is soon sailing off to strange lands.

Armies are awash with paperwork, nearly always meticulously preserved, and this is when – to its detriment – the novel starts to be governed by the disproportionate availability of these archives. The result is a near-exhaustive fictionalisation of the 50th’s marches, skirmishes and encampments; with a pedantry reminiscent of Tristram’s Uncle Toby, we are given the precise dimensions of trench and parapet.

But the siege of Sevastopol, however evocatively fictionalised, can’t help but feel like a history lesson when the story we really want to follow is Ann’s. She is now back in Lincoln and thrown on to parish charity along with those little Mawer ancestors. Thanks to a once-mentioned name in family lore, Mawer is able to posit a link to an unmarried member of that parish committee. He then offers us a choice of scenarios to bring these two together: ranging from the tender to the functional to the purely monetised. But again, Mawer butts in to tell us these are only guesses, somehow undercutting the entire premise of his own project.

These fictionalised elements are never less than credible, if sometimes overdetailed. So it’s a shame that, rather than allowing his characters to grow and interact, as any novel demands, Mawer instead regularly elbows his way on to the page to remind us that, for instance, “this particular rumour happened to be true”. These reminders of his research only serve to disempower the characters and defuse any jeopardy in the storytelling. Things aren’t helped by prose that is a touch too workaday: tones are “dulcet” and hair, more than once, comes in a “shock”.

Mawer himself concedes that the problem with any account of the past is “how to put yourself into the mind of someone who has no idea what is about to happen”. But that is exactly what novelists do – and it can only be done when liberated from the fell hand of history and the hard graft you’ve put in to unearth it.

• Ancestry: A Novel by Simon Mawer is published by Little, Brown (£18.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply