In my country: if it's a split responsibility/fault, insurances are free - no one gets anything. Not sure about British laws. But traffic law says it's his fault - if you have reliable witnesses. If not, you hit him in the back, your fault, no matter what you say. Unless he's stupid enough to admit pulling all the way in front of you quickly. There was a guilty charge in my country with that statement. The guilty driver said: "he hit me in the rear, it must be his fault, I quickly pulled all the way in front of him so hi hit me straight in the rear (LOL)". The driver behind confirmed: "It's true, he pulled all the way in front of me, I had no time to break, so I hit him".