A road-movie for disaffected youth, the book was visually conceived in unexpectedly sleazy surroundings that might surprise anyone who thinks picture books begin and end in the toy cupboard.

The authorillustrator’s own road trip to Las Vegas generated ideas for playing with scale and surreal landscape in a fantasy adventure tale of a little boy who flees the midnight hour in a souped-up toy car. Readers can trace her journey in the book’s echoes of gloriously over-the-top architecture, of ominously empty deserts and of creaking signs swinging in one-horse towns. And the sphinx pose of Cooper’s giant crouching tiger (also a nightdress case) is no accident - it’s inspired by one of the most kitsch monuments on Sunset Strip.

She took to the road carrying the text in her head, not knowing when inspiration for the paintings would strike. Like her other favourite book, The Bear Under the Stairs, Baby took around two hours to write “with a lot of thinking-in-the-bath time . . . waiting for the first sentence that holds the essence of the story” (“Bedtime!” said the Mother. “No!” said the Baby).

As always, the hard labour started with the illustrations, which take a year for each book. Cooper, now 34, is almost entirely self-taught, although she attributes the basis of her technique to an influential art teacher (Angela Lee of William Howard School in Brampton, Cumbria).

“She started me off painting dots, although I don’t think she thought I’d be painting dots forever.” Variations on dappled, mottled, broken effects, often achieved by mixing salt and bleach with her watercolours, have sustained her style ever since. She is delighted to have won a prize awarded by librarians - her local library in Cumbria supported her in her early career, meeting endless requests for illustration manuals and the latest picture books.

A part-time job painting china animals while she did a music degree helped her learn to handle paint. “It took me longer to learn how to write. Writing a picture book text feels like writing haiku, having to be so concise. I learned to use rhythm, rhyme and onomatapoeia partly to draw the child into the text and partly to give me a structure. The hard thing was discovering that any writing I really liked was descriptive and it had to go.

“Now I find colour and texture easy, but I still struggle with drawing. I start by drawing very small, to work myself into the idea.” A miniature storyboard in a notebook stuffed with reminders and triggers and an animation-style character sheet leads her into playing with pace. “This is my main ideas stage. I watch a lot of films and the tricks are like a film-maker’s.” Hence the climaxes, the close-ups, the exciting page turns, the shifts in perspective and the careful balancing of the increasingly exhausted child’s real and fantasy life which delighted the Kate Greenaway judges.

The Baby Who Wouldn’t Go To Bed is her most committed venture so far into the territory of the imagination - a journey that teeters on the edge of the comfort zone.

The Baby Who Wouldn’t Go To Bed is published by Doubleday. Caroline Binch was highly commended for Down by the River (Collins); Christina Balit was commended for Ishtar and Tammuz: a Babylonian myth of the seasons (Frances Lincoln)