From sweatbox buses to a novelty 'dangleway' and fantasy bridges that never saw a brick laid. Boris Johnson’s design legacy in London left the taxpayer with a bill of more than £940m after his eight years as mayor. The Guardian's design and architecture critic, Oliver Wainwright, takes a tour of the worst monuments to Johnson's ego etched across the capital. He finds out what they really cost us then and how much we are still paying for them now