Once the ball is rolling with the Police, they can send a RIPA to Bitstamp which then allows them to 'legally' disclose the information under the data protection act about the account of interest.  That should be pretty straight forward once it's been signed off by the unit's detective.  It shouldn't go as far as a court order unless Bitstamp refuse to cooperate and the Police deem it suitable enough to seize their server hard drives.