My understanding is that News International have basically admitted the hackings took place and have handed over emails, invoices etc corroborating the facts (not suggesting they would have done this without the Guardian investigation. Just that they HAVE done this). So the police investigation is not to determine whether anything untoward has happened. All parties have already admitted this. The investigation is purely about how much was going on and who knew about it and when did they find out.
All of which is irrelavant to the question of actual guilt. So why the delaying tactics? Same, incidentally, goes for Tesco who say they want to see the results of the police investigation before taking any decisions. Why? Who knew what when is irrelevant to the decision they have been askeed to make.