Trading Standards.
https://www.citizensadvice.org.uk/consumer/get-more-help/report-to-trading-standards/
Had a look at their online form, unfortunately only useable by residents of England and Wales. Since I'am German, no chance for me. If it wasn't for that rather small amount of money (not worth the hassle), I would probably take legal action...
European small claims court would work. Best part is they'd have to fight it in your local court not theirs
@FlowRider started
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Trading Standards.
https://www.citizensadvice.org.uk/consumer/get-more-help/report-to-trading-standards/