Contacting a witness through registration details

I was sitting in a queue of traffic when the car behind me went to overtake to drive down a slip road. As they passed they hit the side of my car and left significant scratches. I would imagine their car would have been scratched all down the side of their car and there is no doubt they would have been aware that they hit me. However, they failed to stop and just drove off.

The car behind them (with two people in it) took a note of their registration and passed it onto me, but I didn't get a chance to get the name and contact details of these witnesses. I did however get their registration number as they drove off.

I reported the incident to police and my insurance company. They have since contacted the driver but have advised that I will only get my excess back if they claim liability (which they won't).

The insurance company have told me that they cannot contact the witness via the registration details and only the police can. However, the police have said that they won't be taking any further action as there were no injuries and advised that the insurance company can contact the witness.

So my question is...who is telling the truth??? Can my insurance company contact the witness for me by using the registration details? Surely if they can contact the driver, they can contact the witness if it means the issue will be resolved.

Also, will I only get my excess back if they claim liability, or can I get it back as the accident wasn't my fault?

Any advice appreciated