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Inside Number Ten there was one question they feared most before today's evidence from Andy Coulson - who asked you what and when?

In other words, what checks were made on the man who had to resign as editor of the News of the World over phone hacking before he was hired first by the Conservative party and then by Downing Street.

The answer appears to have been that after giving an assurance when first hired - to Mr Cameron and his chief of staff and his party chairman - that he had no knowledge of hacking, Mr Coulson was not asked for further assurances even when the true extent of the scandal began to emerge.

Downing Street will no doubt be relieved that the Inquiry didn't explore this more and did not reveal more. In advance Lord Justice Leveson had warned those watching not to expect fireworks: those existed in fictional trials, he said, and this of course was not the trial of Mr Coulson.

It was though the beginning of day one of a new phase of the Leveson Inquiry: one which poses real dangers for the prime minister, the government and for politicians in both main parties. Today's appearance by Mr Coulson will be followed by ministers, the prime minister, his predecessors Tony Blair and Gordon Brown and, tomorrow, by Rebekah Brooks who poses a real threat to David Cameron's reputation.