The Sale of Goods Act is completely irrelevant. It was replaced by the Consumer Rights Act in 2015.
Under that, if it fails and you return it in the first 30 days, you are entitled to a full refund. If you discover the fault in the first six months, the retailer has one opportunity to repair or replace, after which you are entitled to a full refund (or partial refund if you want to keep it). If it's more than six months, you have to prove that the item was faulty when you received it. Rather more information available from https://www.which.co.uk/consumer-rights/regulation/consumer-rights-act. Mrs Zero used to be head of customer services at an online clothing company, and I have had my ears bent about this.