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!
If the car park is covered by cameras you should have had a word with security to see if you could use the mobile to record the CCTV playback. Depends on how nice you are when you ask them. Happened years back on a complex near me and they let me view despite there being a form for a full copy. As it was it was a scally tw4t who hood up keyed the side. Jealousy. And Nowt I could do but at least I knew before paying for a copy of the footage.
And old buggers don't always know they hit you either. Other half had this happen although thankfully no damage whilst waiting for me to come out of the chippy with our Friday night fish n chips. He looked dazed and shocked apparently and was shaking as he didn't realise he reversed into her.
Glad all sorted for the OP
