are you a mechanic?

if the answer is no, then it's up to them to have sold you a saleable vehicle and rectify any faults within the first 3 months as stated on your bit of paper than you signed

if the answer is yes, they you may be a bit fucked if it goes to court as they could then say you were aware of the problem in the 2.5k miles you used the vehicle

don't be a mechanic, be a member of the public who has bought a lemon and then been fobbed off by the garage that sold it which is why you are going to court to recover your money

although if they say 50/50 you can be sure they will bump up the bill to make it 200% of what it is so that when you "only pay half of it" you will be thankful