With his latest foray into a parallel universe, Terry Pratchett leaps into the Trousers of Time with Johnny Maxwell and the gang who first explored computer-game space in Only You Can Save Mankind.

In book three of the series, Johnny’s home life is still bogged down in “Trying Times”, and he and his mother are still living at his grandad’s. Too much is uncertain, too much is changing, he feels unsafe in the present. But instead of swooping into a sci-fi future or dreaming himself into the kind of fantasy world that readers of Pratchett’s Discworld series enjoy, Johnny is sent back to the past.

Blackbury’s Paradise Street in wartime is a place where jolly evacuees play in the street, old men go off to work their allotments in “special old man’s allotment trousers” and the woman in the tobacconist’s calls Johnny’s friend Yo-less (the only black boy in fiction who’s so uncool he wears school uniform) “Sambo”. Mrs Tachyon, the local bag lady, is the mysterious link between past and present. Her Tesco trolley - the one with the creaking wheel - is the time machine that transports Johnny and the gang to May 21, 1941.

Mrs Tachyon bears a strong resemblance to the Discworld witches. Even Mrs Tachyon’s cat, Guilty, has come from the same spawn-of-the devil litter as Nanny Ogg’s Greebo. But here the witchy figure is only a catalyst for the action. It is Johnny and his friends who blunder to the rescue when a German plane bombs Blackbury.

The plot takes off with the trolley on an exhilarating switchback swirl through different but interconnecting versions of the same two days, one in the Nineties and one in 1941, making a story complicated enough to hold the attention of even older teenage boys for whom at first sight a book with a 12-year-old hero might seem too young.