Ensure that the V5 is correctly registered in your name, if he felt like it, or anyone else for that matter, i believe you can apply to dvla for a v5, even if the vehicle isn't/wasn't registered in your name, if he knows the reg no, and the chassis number, i 'think' thats all that is needed, so, remove the plates, although if he's dodgy, he's most likely already noted down the number...
It still amazes me the number of people even now selling chassis's on ebay which clearly state, no v5, ideal hybrid etc, things like this are totally illegal, once a chassis has a number, thats it, thats its number for life, unless the dvla are informed of a chassis swap for a NEW one, and a new number issued.....