I just figure that if/when it goes to court that if you've done over and above what's reasonable, then the court is highly likely to look more in your favour. It's a pain, and it's wrong, but it's also easier long term.
Send them a letter by SPECIAL delivery (not recorded, you want proof they signed for it) that you are rejecting the vehicle under the CRA 2015, and give them 14 days to give you a refund. In the mean time, send the car back to them. They have precisely nothing to stop them issuing the refund then.