By City AM reporter London Mayor Sadiq Khan is urging ministers to end the “everyday extortion” of unfair service charges for leaseholders. He is demanding legal powers forleaseholders to withhold service charge payments where the justification has not been set out in writing. The City Hall incumbent’s intervention comes less than a week before the May 2 mayoral election in the capital, where he is seeking a historic third term in office. He said: “If leaseholders face large increases in their service charge without it being clearly explained what it’s paying for, they should be able to do som…