http://www.amazon.co.uk/Children-Chieftain-Betrayed-Michael-Wills-x/dp/1781324018
This is an adventure story about a real place, a town called Birka. It was once one of the greatest trading centers in the Viking world. But the wealth of the town on the Island of Birches attracted some who did not come to trade. When the town was raided by the most fearsome of Vikings, the Jomsviking, many of the islanders are captured. The children of the chieftain are forced to take action and lead their friends in a desperate attempt to rescue the captives. But not all of their allies are as loyal as they should be. The brave orphans are betrayed and find themselves in grave danger of captivity and risk being sold into slavery.