For me, the rule of law must trump all - the moment we sidestep that, it's the moment an already unfair (for the common man at least) legal system instantly becomes unacceptably weighted towards the state, bad policing and loss of the "justice" from the justice system.

In your situation, the judge would almost certainly be expected to throw out the evidence. I'd almost always prefer one guilty person go free than set a precedent that allows dodgy evidence and corrupt practices to convict innocent people. Plus - if the police /really/ know he's guilty, he'll be watched, and probably caught again.