Montmorency, known only by his pseudonym, is now a middle-aged man whose seedy, criminal alter ego, Scarper, is almost a thing of the past. A member of Bargles Club and an amateur spy for the Government, he lives in great style and is to all appearances a perfect Victorian gentleman. His great friend, Lord George Fox-Selwyn, was, however, cruelly murdered by Italian terrorists in Montmorency and the Assassins and now, while Queen Victoria lies dying, those who remain are bent on revenge.

They have pressing personal problems. Montmorency is agonised over whether he should acknowledge his illegitimate son Tom, believed by others to be Lord George’s child. Kindly Dr Farcett, the surgeon who saved Montmorency's life when he fell though a skylight as a thief, is now unhinged by guilt and grief. Luckily, this provides a perfect cover for the young Frank Fox-Selwyn to pretend to check in to a lunatic asylum, where Dr Farcett can be the true patient and Frank can pursue his uncle’s killers. But the assassins are as cunning as they are secretive, and there is a long transatlantic chase in store, with the attempted murder of the President of the United States brewing. The band of friends will need all their cunning, as well as the help of Scotland Yard, to prevent more murders.

Secret codes, aristocratic heroes, unambiguous villains and swift changes of scene are all tried and trusted elements of classic boys’ fiction and Updale employs them with wit. If you can imagine the world of Buchan, Anthony Hope and Sapper, purged of racism and plunged into an earlier era, you will get a fairly good idea of the good, clean fun to be had in a sewer.

In many ways, these don’t read like children’s novels at all, for all the protagonists are adults, engaged in grown-up dilemmas. It’s this very male, slightly snobbish, clubby world that boys enjoy reading about, however, and although it could do with a bit more period detail the introduction of real-life people such as Roosevelt and Verdi is an amusing feature of the series.

Inevitably, the best bits are always when our hero switches from gentleman to street-rat and wonders which is his true identity. I missed the humour and the Buchanesque disguises of the earlier books, but the short, fast-moving chapters never flag and the melancholy whiff of real grief deepens Updale’s considerable gift for character.

Four novels have taken us from Montmorency’s youth to middle age and this one ends on another cliffhanger with a gun being held to the back of his head just as he is about to tell the women he loves the truth about his life. Will he live, or die a liar? Only another instalment will tell.

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