The Last Castaways

On holiday in Saltbottle, Grandfather and Roo discover the Unsinkable, the boat which took them to the North Pole in the first book, lying in a sadly dilapidated state and up for sale. At the auction, the short-sighted auctioneer accidentally sellsthe boat to Roo - sold to the old lady in the brown fur coat! Now they have to find the money and the Captain suggests a fishing trip to the Great Cod Banks in the Forgotten Sea.
Off they go, but the trip ends in disaster when the boat sinks and Grandfather and Roo are cast away on a desert island. They find their own Man Friday (called Tuesday) but he turns out to be a film director staying in a luxury hotel. Meanwhile, the Captain has found the treasure and rescued the Unsinkable.