Given that catching this person is very important, when you next see him drive past I would recommend that you note down the registration and pass it on to the police - local safer neighbourhood's team 020 8721 2723 or national non-emergency number 0300 123 1212.

It sounds like you have treated this individual with the correct amount of caution and it could be dangerous to attempt to gain information more directly or to play along with him in any way.

However, as you point out, this person could be completely harmless. I hope he is for your sake, but then again it would be good to get this person off the street and prosecuted.