Isn't this what a small claims court is for? A customer complains to a dealer for a fix. They disagree on whether the dealer is liable under law or whether the customer is taking advantage. They then go to a small claims court for a judge to decide based on evidence.
I don't know whether £300 to £600 injectors are covered under consumer law for an older car. But if it is indeed covered by law then the dealer should consider that before selling a cheap old car. Buying cars from a dealer is more expensive than buying them from a private seller exactly because buying from a dealer is meant to offer a degree of convenience and protection over and above a private sale. Personally I'd prefer to buy privately as I don't see much value buying an old car from a dealer. But there is a price difference between the two for a reason.
How can a judge that isn't an engineer/mechanic/engine builder/production staff have any clue, they won't. And how can someone say something put an estimate let alone a guarantee on a 14 year old mileage mileage part let alone to last till next month, you simply can't.
They don't take the car and engine apart, nor do the flow test the injectors etc etc etc. It is a serviceable item based on mileage/wear and tear it's as simple as that, it's just bad luck.
Imagine buying a 2-3k car which that is marked up by a dealer, and then expecting them to basically pay 2-3k for brand new injectors/fuel pump/pressure regulator and god knows what else could be the issue (potentially) via a claim in court.
I'm not saying the OP is at all doing this, my rant has purely been based on how slack/vague this law is and the fact it'll be handled by a judge that isn't at all qualified in the field, relying on some jumped up solicitor's opinion/3rd party garage...
Which again is stupid as you'd have to have taken the engine apart/tested the injectors at a 3rd party to prove any of this (which you won't as they can work fine one minute then fail tomorrow being old/X mileage) so again no way to blame the seller of the car nor the owner.
So thus a silly vague law based on the hope the right judgement is handed out in court...
And yes I buy privately and put the saving into doing all the serviceable/age related/known faults of X car I'm buying that I'd obviously have researched heavily vs buying blindly, just with anything in life...
Imagine taking someone to court over something you know full well is a known weakness on that car or is known to wear out over X miles/type of driving, knowing this then still screwing over the seller, scummy asf! Again not saying the OP is, but it still falls into why this 'law' annoys me.