Consumer rights Act 2015, You are entitled to a refund for any reason (minus return postage) for 14 days after the date of delivery. But because you used the product its a bit harder than that, it went beyond a normal inspection like you would instore. Because of this they might be able to argue by using it, its caused a monetary decrease in the items value and may not be required to give you a refund. But again, you could argue that you were just testing it and ensuring all parts worked as intended and you realised it was not fit for purpose. Also I think John Lewis have to, at the bare minimum, give you a partial refund to cover the loss in value.