Unjustifiable Declining of claim by outsurance

Outsurance is the worst insurance ever. One of their client overtaked me towards a traffic light and he cut in front of me, where he scratched my front fender and bumper. I did third party claim and requested no claims letter from my insurer. They declined my claim, saying I should have slowed down faster and allow him to get infront of me. I was slowing down at that time cause I was approaching a traffic light. Their clients also did attest to my statement. Saying I was slowing down and he decided to overtake.