car accident- whose fault?

[YoursTruly]

it is mostly his fault but also, you should have looked behind you to make sure no cars were coming.

 

[Anna]

i live in the uk.on the m4 motorway the traffic was stalled and i was on the third lane. i wanted to move to left lane and started giving signal.the car on the left created a sapce for me and then i did all my checks,checked all the mirror and blind spot and started maneuvering slowly.now suddenly a motercyclist came along in a very highspeed filtering the lanes and hit my car on tangent at the front left and my bumper came off. he wasn't injured neither his moterbike( for obvious reason because my car was almost @9-10 mph). he agreed his fault partially but not completely and when i checked with local garage they gave me a quote to repair the damage for 900 pounds.the faulty guy agreed to pay 400 pounds which was not convincing so i went to my insurance company for claim but the representative highly discouraged me to claim and said this claim couldn't be settled because that guy's fault can't be proved and my insurance company will payout and i will loose my ncb as well.this is very frustrating. i am being a victim where i have not done any fault.can you throw any light...like whose fault is it under uk regulation.

 

[TomJ]

this is completely the other guy's fault you followed regulation he wasnt aware of his surroundings