My Uncle once went to overtake a car with its left indicator on in a 100 zone, only to have them turn right in front of him. He had his 5 teenage kids and wife in the car at the time. Fortunately everyone survived, but it could have been worse. Seat belts accounted for a lot of that, because his car rolled.
This is where dash cams are coming into their own. The Police will quite happily examine video evidence. Especially if you tell them you are prepared to give evidence in court.