SirPrimalform: If it was sold as working then you're entitled to a full refund including postage (and the cost incurred sending it back).
SimonG: mostly this, actually, the seller has either the right to repair it or to send you a new one. Whatever he does, he does have to cover the shipping. Usually you can't demand a refund until he ultimately failed to fix the mistake. That is EU law, so UK law might be different in the details.
I think the seller probably does have the right (in the UK) to fix it or send a new one, I was just assuming they'd be unable to. Either way, you're entitled to the money it cost to send it back.