Try the seller first
The Ombudsman says that for a chargeback you often need to show you have tried to resolve the problem with the supplier. Keep a record of what you asked for and when.
Ask your card provider to consider both routes
Explain the problem and ask the provider to consider both chargeback and section 75. Chargeback time limits are short, so do this promptly.
Gather your evidence
The Ombudsman suggests receipts or invoices, contracts and terms, marketing material or screenshots, emails, photos or videos, and any independent reports.
Escalate if you need to
If you get no final response within the time limit (eight weeks for most complaints), or you disagree with it, refer the complaint to the Financial Ombudsman Service within six months of the final response.