Lestrade and the Giant Rat of Sumatra

Book seventeen in the Inspector Lestrade series.
Everybody, they say, has a book in them. Retired Chief Inspector Walter Dew certainly did. And it took him back to the good old days, when coppers lived in station houses, that nice Mr Campbell-Bannerman was at Number Ten and Britain had the biggest empire in the world. But, under the streets of London, something stirred. More than that, there was a muttering that grew to a grumbling and the grumbling grew to a mighty rumbling. Then out of the houses, the bodies came tumbling!
Superintendent Sholto Lestrade, with Dew by his side and the rookies Bang and Olufsen in his wake, must go Below to face their demons, to find a murderer whose machinations will upset the infrastructure of the richest city on earth.
Will any of them live to tell Dew's tale? The tale of a rat.