The Lost Island Adventure

When James and Jessica are on a family holiday in the far north-west of the Scottish Highlands, they meet Tarben Macaulay, a boy their age who claims he is a Viking, although he seems friendly enough. Just off the coast where they are staying in Puffin Cottage is Arraig Island. When the family comes back from a shopping trip, the island is no longer there. That mystery is quickly solved, but Arraig Island becomes a dangerous place as Jessica and James, helped by Tarben, make two exciting discoveries. Are the strange couple with the campervan really birdwatchers, or do they have another interest in the island?
This is the sixth Two Jays Adventure. They can be read in any order, although each one goes forward slightly in time.