If you shopped in store you are not actually guaranteed a refund. Many retailers will give you an exchange or a gift voucher, but they do not have to so check return policies before you buy and keep receipts.
You get 14 days to change your mind after receiving your order, and another 14 days to send it back once you have asked for a refund (read this for exceptions). Your refund should be paid within 14 days of the retailer receiving your return, and include the cost of standard delivery.
If something you have bought is not of satisfactory quality, as described by the seller, fit for purpose or does not last a reasonable amount of time you are entitled to a refund. This is your right whether you shopped in person or online - even if a retailer's refund policy says differently.
You have 3 weeks to return faulty goods to the retailer (not the manufacturer, even if they tell you otherwise) for a refund, after this you may need to settle for a repair or a replacement.
If you are refused a refund, repair or replacement, then you can file a complaint with the company.
If this does not make any difference, then try contacting the Consumer Ombudsman, who will aim to help you resolve your disputes within 10 working days or advise you of further options.
You can also use the Resolver website for more help making a shopping complaint.