Burden of proof - on whom?

Could someone confirm who bears the burden of proof in this situation.

Say a customer purchased a car from a garage, one year later car had a problem, the garage fixed it but the customer is refusing to take the car back stating he thinks the problem has caused irreparable damage to the vehicle and he doesnt feel it is safe to drive.
The problem is a common enough one and there has been no damage and it is perfectly fine.
Customer has contacted a solicitor and is seeking a full refund - Sale of Goods - not fit for purpose/merchantible quality etc etc.
They are requesting the garage provide an independant report stating the vehicle is roadworthy.
In my understanding the customer bears the burden of proving the car isnt roadworthy as he is the one claiming this is the case, so surely he should be the one to cover the cost of the report and not the garage.

As he instituted the claim should the burden of proof be on him?
Thanks.