The Sale Of Goods Act assumes that any product that develops a fault within the first six months was faulty from new. Moreover, it requires that goods must be as described and sufficiently durable and well-made to be fit for the purpose. As you notified the seller within the six months, they don't have a leg to stand on. Complain to Amazon and they should intervene for you. Saves a lot of hassle if they just get your money refunded, but you can also complain to Trading Standards on their website.
The seller must either replace or repair the cable, or else return you to the financial position you would have been in, had you not purchased the item at all. This means a full refund, including postage both ways. If the seller does not tell you to keep the cable or dispose of it, the law requires you to return it.
Like I said, though, just get Amazon to sort it out. Hope this helps.