Send a formal written demand for your refund to the dealer. Your letter should:
(a) show your name and address ;
(b) show the name and address of the company. (I suggest sending it to the registered address of the company, rather than to the local dealership. You can find that here:
https://www.gov.uk/get-information-about-a-company ) ;
(c) be dated ;
(d) state the amount you're demanding ;
(e) state the reason for your demand ;
(f) state that if you don't receive payment within a specified period (I suggest 14 days) you will commence court action against the company, using the small claims procedure ;
(g) be sent by Royal Mail's 'Signed For' service.
If you don't get your money commence your court action against the firm by using the online service here:
https://www.moneyclaim.gov.uk/web/mcol/welcome