CAB is the better first port of call. They will help with details of your options when they see the details. Advice on how to go about it within the legal framework of Consumer Rights law.
It can be a minefield with the usually often main problems being from mis selling by false claims and descriptions to basic “fit for purpose” requirements.
Often, getting the evidence to prove the seller is in the wrong can be difficult. If the dealer doesn’t sort it then legal action via small claims is often the result. Even getting a judgement in your favour can still mean no money from the dealer.
Andyrich is spot on with the post above. Do that asap. Only deal in emails, written contact to provide a paper trail.
Follow the process TO THE LETTER.
Persevere and don’t be a softy. Stick to timescales and don’t accept less than satisfaction.
