Start with a formal written complaint that cites your rights under the Consumer Rights Act 2015 and sets a reasonable deadline (typically 14 days) for a response. Keep a copy of everything sent.
If the trader remains unresponsive, consider these escalation options:
Section 75 of the Consumer Credit Act 1974 — if you paid by credit card or used a linked finance agreement and the purchase was between £100 and £30,000, your card provider or lender may be equally liable.
Chargeback — available for debit card payments, though this is a bank discretion rather than a statutory right.
Trading Standards / Citizens Advice — can advise and may take action against rogue traders.
Small Claims Court — for amounts up to £10,000 in England and Wales; relatively low cost and no solicitor required.