David Cameron hires a man to deal with press and public relations. He hires a man who used to work for News International as a journalist, a smart move. Communications Chief is a high profile role, just ask Alisdair Campbell: despite being given warnings about about employing this former journalist, David Cameron goes ahead. So long as he never asked said former journalist "so, is there anything I should know, anything from your past that might prove embarrassing or, who knows, maybe incriminating...?" David Cameron is not party to any crime.
David Cameron has a neighbour. This neighbour IS a journalist with News International. This neighbour is also a friend. The friend of David Cameron gets a new horse. The friend is understandably very proud and invites people round to see it. When David Cameron went round to have a ride on it he was not aware that it was a police horse lent to said journalist. Meanwhile, in a totally unconnected set of events, said journalist is suspected of bribing civil servants and police officers in return for information. So long as David Cameron never said "so, I see you have a new horse, that's interesting, where did you get it from...?" he cannot be party to any corruption.
David Cameron likes to eat, who doesn't? But he is a busy man. He has to have someone to arrange dining partners for him. Who wants to dine alone? But the people who arrange dining partners for David have a preference for Tory party donors suggest that the more you donate the better your access to David Cameron is and that £250,000 is the kind of money you need to sway government policy. So long as David Cameron never says, "hey, you've done so much for me, and you've been such good company while I've swallowed these canapes and drunk this champagne, is there anything I can do for you...?" he's not a corrupt scumbag for hire.
In other news David Cameron lives in the Land of Chocolate.