You would find it very hard to reject the car as you've done significant mileage, CRA is going to be focused on a car you receive and is immediately faulty. This sounds entirely like a warranty issue that's being dealt entirely in line with your warranty agreement. You would be trying to explain that the car was of insufficient quality from new as this fault was present in a dormant state when you purchased it, I really doubt you would get anywhere.
Does your warranty agreement have any terms that define that it will extend under these circumstances (it doesn't) so that's also simply unlikely to happen.
Appreciate it's frustrating, but you need to be realistic about your rights. 3000 miles in this is not a new car.
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Rejecting a new or used car: your car consumer rights explained | Auto Express