What does the latest No. 10 resignation mean?

<div>Munira Mirza, Downing Street's head of policy, has resigned over Boris Johnson’s Jimmy Savile attack on Keir Starmer.<br> <br> Katy Balls talks to James Forsyth and Fraser Nelson about how the defection of one of Johnson's oldest allies will affect an already turbulent No. 10. <br> <br> <em>'This does look terminal.'</em> – Fraser Nelson</div>