Claim. right away.
Many moons ago i had a simliar situation but i was in the car.
I didnt do what this guy did to you but by indicationg and turning my wheels full lock, while in stationary traffic, made the guy comming up behind on a Transalp loose it and take a tumble. It was pouring with rain at the time and i jumped out to help him up and he addmitted he hit the brakes too hard and locked the wheels. he knew i wasnt going to turn into his path because it was nose to tail, stationary rush hour traffic.
But my insurers paid out
Direct line they were at the time. My argument was that there was no collision and he by his own addmission braked too hard for the conditions and lost grip.
I was riding back then too so you'll understand that i wasnt being a dozey idito like this gy sounds to have been, as i was aware the bike was approaching and simply turned my wheels while i had my foot on the brake so he could see i wasnt about to turn.
I often replay that incident in my mind and try to put myself on his bike, seeing what he saw and wonder would i have hit the deck like he did.
Err, sorry, ramble over..... point being there dont have to be a collision for you to claim. Legaly they see it as his behaviour was enough for you to take drastic action and your binning it was a direct result of him.
hth