The London Eye Mystery by Siobhan Dowd #1

When Ted and Kat watched their cousin Salim get on board the London Eye, he turned and waved before getting on. After half an hour it landed and everyone trooped off - but no Salim. Where could he have gone? How on earth could he have disappeared into thin air? Since the police are having no luck finding him, Ted and Kat become sleuthing partners. Despite their prickly relationship, they overcome their differences to follow a trail of clues across London in a desperate bid to find their cousin. And ultimately it comes down to Ted, whose brain works in its own very unique way, to find the key to the mystery. This is an unputdownable spine-tingling thriller!