It is now in the hands of the insurance companies, i am assuming because it was a demo, you where insured via the garage, is so you just need to pass on the details.
With regards to any excess, the garage should have pointed this out before u took the car, or maybe you even signed some papers ? If you are responsible for any excess, you will of course be able to claim it back at a later date, providing of course, that the other driver was to blame for the accident.
~Good luck ..