I agree.(Original post by RobMac93)
Did you take photos of the damage? Always a good idea to make sure someone isn't going to turn around and blame you for extra. Am I right in saying you were following him then he braked sharply to turn off the road you were on and into a side road, and you hit him up the chuff? If so then as far as I can see it's all blame on you, and if he goes through the insurance they'll likely agree.
The £860 seems like alot, but if you can get it together it may be worth it. If you claim there's a decent chance your mum is going to lose at least some of her no-claims, and I'm guessing it will cost you significantly more than £860 over the next few years just because of the claim. If you can't afford it, the best you can do is tell him that and see if you can work something out. Unfortunately he's got you by the balls with this and you'll probably have to do what he wants.
who are introverts?