Does not matter who's fault it is- you claim and you lose the NCB- when they calculate the amount they contribute to the damage, its based on the sum insured verses the market value of the car. If both are about the same then should pay 100%- didnt you ask them why they deducted 10%?
Hmmm 60k for 2.3 mil is high. I'm with them and the proportionate amount I pay is lower. There are certain non essential elements that you could remove from the policy in order to reduce the cost- advice you go through the details and check