Send a letter by recorded delivery to this address:
Your letter should clearly show that name and address, together with your own. Word it as follows:
"DEMAND FOR PAYMENT
On (date) I paid £xx.xx for a product which I have not received. I therefore DEMAND immediate repayment of the said sum. TAKE NOTICE that if I do not receive the said sum within 14 days of this notice I will commence LEGAL PROCEEDINGS to recover this amount together with my LEGAL COSTS
D. Maximus"
If you don't get your money back, start a legal claim by using the online service here:
https://www.moneyclaim.gov.uk/web/mcol/welcome
Chris