Speak to Citizens Advice and they will be able to point you in the right direction.
I’ve had something similar years ago with a car that had a fault and was under manufacturers warranty and they wouldn’t fix it despite being aware of the fault. After a couple of letters and some assistance by CAB they backed down and got it sorted.
If you were to take them to small claims I’d imagine that they’d get laughed out of the place trying to argue it. It dumps coolant out when you’re driving, it’s not meant to therefore it’s faulty. Once they admit that they’ll not have much option other than to fix it or refund you I wouldn’t think.