Mayor Sir Sadiq Khan has been accused of “stonewalling” questions about whether London has a problem with so-called “grooming gangs”.His Tory rival Susan Hall made nine successive attempts at Mayor’s Question Time to ask Sir Sadiq whether there were gangs operating, or previously active, in the capital.But Sir Sadiq repeatedly asked Ms Hall, the former Tory mayoral candidate, to clarify what she meant by grooming gangs, claiming: “I’m not clear what she means.”