Gunpowder plots, political skulduggery and intrepid Cockney foundlings have always exercised Joan Aiken’s imagination. Cold Shoulder Road, the latest in her James III series (the first was Black Hearts in Battersea in 1965, now being serialised on BBC1 on Sunday afternoons), gives further vent to her fantasies and finds the Twite family still going strong in a never-never land where steam trains chug along the Channel Tunnel packed with contraband mammoth tusks destined for the false-teeth trade.

Aiken’s achievement throughout the series has been the dextrous juggling of anachronisms with period detail to create a time frame that encapsulates the whole of British history. Cold Shoulder Road is no exception.

Arun and his cousin Is are the latest Twite children to find themselves immersed in sinister intrigue. They return to Folkestone after a stint in the northern coalmines to find their home town terrorised by the Merry Gentry, a vicious gang of Tunnel smugglers. Arun’s mother, Ruth, a member of a peace-loving sect called the Silent Folk, has fled amid rumours of witchcraft and child abduction, while the Folk have disowned her for defying their leader. She eventually turns up in the hull of a frigate suspended in the branches of a giant oak. With her is a mute two-year-old called Pye, a silent witness to the Merry Gentry’s nefarious doings.

Language, or its absence, is central to this book. The Silent Folk worship through the Holy Silence which “wraps each one of us in a thick, thick quilt of solitary wonder and mystery”. Is has a more pragmatic view: “It’s a scaley notion . . . Why did we ever invent words . . . if we ain’t to be allowed to use ‘em?”

Boisterous use of language shows Aiken at her best; in an idiom that combines Edward Lear with the Artful Dodger, everything is “all ruggy” unless it is “numbjumbous”; rain falls in “stair-rods” and the complex plot is summed up thus: “In Fokston now things is turble bad. The mery gentry what smugles mamoth tusks is makin fokes lives a Mizry. Two many fokes getin kild. Too meny fokes scared to speke.” The convolutions of the story become so coiled and the action so frenzied that the reader is almost hurled at full pelt from the roller-coaster. Luckily the compelling force of Aiken’s narrative drive holds us in place for the inevitable Twite vindication.

A Handful of Gold gathers 16 of Aiken’s short stories to celebrate her 70th birthday. Although tales of family life, cat-flaps and “wet sandy swimming trunks” curb her fertile experimentation - prosaic dialogue does not suit her - there is more than enough fairy-tale symbolism to sustain the collection.

Gold, as a manifestation of light or as an object of adult desire, is the double-edged metaphor that shines through these stories. Her favourite device of the abandoned child finding his or her way through a hostile world, aided only by magic roses from Saturn or mysterious rocking donkeys,is best exemplified in “A Harp of Fishbones”. Nerryn, searching for her father, lulls circling vultures and melts death-dealing frost with “a curious watery music, like the songs that birds sing when it is raining.”

The best of these stories celebrate the life-affirming powers of music, magic and the inexhaustible exuberance of children. This collection shows that they have inspired Aiken through 30 years of writing and that her ingenuity has never wavered.