In the event that a refund is issued, the seller is legally able to deduct an amount to reflect the usage costs incurred whilst the vehicle was in the buyer’s possession, such as the extra mileage on the car. Any amount deducted for that must be reasonable and fair.
An important aspect of the Consumer Rights Act 2015 is that there is an assumption that any issues complained of, which have become obvious or developed within the first 6 months of buying the vehicle, were present at the time of purchase. If the seller disagrees that his was the case, it would be up to them to prove otherwise, if challenged in court. On the other hand, any issues which develop more than 6 months after purchase, are assumed not to have originated at the point of sale and it is for the buyer to prove otherwise if challenged in court.
Once a decision has been made on which of the above rights to pursue, the seller should be contacted, preferably in writing, to discuss that with them. If they refuse to discharge their legal obligations under the Consumer Rights Act 2015, a formal letter before action should be sent, asking for the desired resolution and making it clear that legal action could follow through the courts.
In the event this matters needs to be taken further, the following are the relevant links:
A report to Trading Standards can be submitted first: https://ssl.datamotion.com/form.aspx?co=3438&frm=general&to=flare.fromforms
Afterwards, a claim can be pursued in The County Court: https://www.gov.uk/make-money-claim