Your Mum saw it, therefore she is a witness. Go to the Police station, and they will take all the information off you. How do I know? It happened to me on Friday!
I borrowed a mates Civic Type R, drove into Fareham to get my hair cut, and left the car in the town centre car park. Came back to a nice little note on the windscreen, saying "woman driver in White Golf

Reg Number ................... hit your car and left.
Front bumper/splitter and wing has paintwork damage. My mate was relatively understanding about it, considering! so we went to the local police station, which incidentally backs onto the car park!
They gave us a form to fill in, and as its hit a hit and run incident, they will follow it up. They said though, that as the witness never left their details, its unlikely (unless the guilty party admits it) that their insurance will pay out. There are cameras in the car park, but they are on 24 hour loop, and unlikely to be viewed before the form would be processed!!!

Good luck, and hope you see the old prick off for as much as you can!