When I wads young I remember listening to a poem by Dylan Thomas that was quoted in an audio version of Matilda by Roald Dahl. I can only remember it very sketchily but have never been able to track it down. Can anybody help?

This may well not be accurate, but you get the idea...

Never and never,
My girl riding near and far
In the land of te half-spun tales
and spelled asleep
..........
Fear or believe
That the wolf will leap my dear
..........
To east your heart
In the house in the rosy wood.

fingers crossed!

It is from "Country Sleep," by Dylan Thomas. You did a good job of remembering the first part:

Never and never, my girl riding far and near
In the land of the hearthstone tales, and spelled asleep,
Fear or believe that the wolf in a sheepwhite hood
Loping and bleating roughly and blithely shall leap,
My dear, my dear,
Out of a lair in the flocked leaves in the dew dipped year
To eat your heart in the house in the rosy wood.

(See [expert.ics.purdue.edu] for the rest of it.)