Drunken driver.What would you do?

Both strangers to the area we left Minehead for Bridgwater about 4 pm yesterday along the A39. Just after we left a Blue/grey Volvo pulled out in front of us. The driver was driving erratically swerving from side to side and braking for no particular reason even on clear open roads. We guessed that he was probably drunk and a danger to himself and maybe others.For those who don't know this road it is about 30 miles long. A winding single lane road with little or no chance of overtaking and ping through a number of small villages. We followed behind him for nearly a hour until reaching Bridgwater when he suddenly turned left through a red filter light at which point we lost him. We both have mobile phones. What should we have done? What could we have done? What would you have done?