What caused Geidt's flight?

Lord Geidt became the second ethics advisor to leave Boris Johnson's government last night. It seems like Chinese steel tariffs was the straw that broke the camel's back, but it is clear that he hasn't been happy in the position for sometime. Will the Prime Minister be able to find anyone to pick up this poisoned chalice? 

Max Jeffery speaks with James Forsyth and Katy Balls.