The Flitspace by Stewart Cable

On their Scottish holiday, Ru-um and his little brother Cali go missing during a walk to an ancient, ruined broch. They were last seen with Amelia, a mysterious girl, and were eventually found days later unscathed, but with Cali mysteriously aging faster than Ru-um.

The story breathlessly follows the boys as Amelia takes them deep under the broch to the Flitspace - a gateway to another world identical to their own but completely uninhabited, except for the descendants of a small tribe who fled from Scotland one thousand years ago.

But Amelia has yet to reveal the full truth about her world and the peculiar effects of the Flitspace on aging.

The boys soon clash with Amelia's formidable brother Mor-Ri and his mysterious counsel, the enigmatic Minister.

With Amelia’s help, their own knowledge of the landscape, and the discovery of megafauna not seen in present day Scotland, the boys attempt to thwart Mor-Ri’s plans and escape back to their own world.