As he sat in the shepherd’s hut, surrounded by piles of his unwanted memoirs, David Cameron could be forgiven for wondering how it had come to this. He was having to explain his contacts with the government,  his former colleagues and pals – on behalf of Greensill Capital, the financial services company. Lex Greensill, the firm’s Australian founder, had become a mate and when Cameron left Downing Street, Greensill offered him a job and some shares, too.