Right, the DVLA will provide you with a vehicle owners details if you give them a reg number, but only for a legitimate reason! (E.g. you own a small shop with a private car park and someone parked there who wasn't using the shop so you wish to send them a fine through the post - It has happened to my dad before!)

Your case may be a legit reason, but it is best to ask them if it is before telling them the plate/your name

You will then need CCTV or some impartial witnesses if you want to have a chance of a case against him.