Contact them, say you were not in the area, and get them to provide a photo. Not sure about the comments on VIN , as the police can only go on the reg plate - they won't have the VIN for the other car.
I had a NIP recently, in Hampshire co-incidently, and asked for the pic - you could clearly identify the driver (me) and wifey in car. That was a mobile speed trap.
The offending car may well be another GTI in the same colour - that's why these muppets copy the plates.
Hopefully the pic will prove all you need, in sufficient detail - they sent it electronically as I remember, so was quite quick, but resolving this one could take a while
Good luck