Paddling in dangerous waters?

Plebgate rumbles on.  Rather surprised to see the Prime Minister come out so forcefully here:

Cameron said: "A police officer posing as a member of the public and sending an email potentially to blacken the name of a cabinet minister is a very serious issue. It does need to be seriously investigated."

Given that the officer concerned has been arrested, it seems to me that the PM is treading dangerously close to contempt of court.  Bearing in mind Cameron's expostulations, how can the officer be given a fair trial?