I'd bought them online, but I took them to the Shoreditch shop as it's not far from me. I had to log in to my account on their terminal and show them the transaction, and they did the rest. Got a refund. Have since bought the crochet mitts and they're great.
That happened with my classic mitts too.
Under the consumer rights act, if you return faulty goods within six months of purchase you don't have to prove whose fault it is.
https://www.citizensadvice.org.uk/consumer/somethings-gone-wrong-with-a-purchase/return-faulty-goods/