Car accident question?

Here goes.sorry this is a long description. yesterday while driving home another car pulled out in front of me from the ENTRANCE to a petrol station. I swerved missed him but caught another car. Myself and the gent in the car i hit pulled our cars onto the pavement and myself and my mother hopped out to check on him as he had an estate car and i feared that children where inside, luckily that was not the case and everyone was fine. Yet during this the car that had casued the accident just took off. myself and the other driver who was very friendly exchanged details and found out that the garage has CCTV, so awaiting to see if the footage has his reg and proof that it was his doing. Now knowing my luck both of these things wont happen. I am in contact with the other driver and if worst comes to the worst i will need sort things. this is my first accident and i currently have 4 yr no claims on my insurance. the other driver has suggested that if i need to pay and its near the excess to just settle it between ourselves and not involve the insurance. I feel i can trust this man and I am more than sensible to get things in writing and have statements e.t.c just in case. so heres my questions if things go this way do i need to inform the insurance companies at all, and do i need to mention this when filling out any car related documents/re-insuring? may have some follow up questions, if any other details are required please ask.