Ebay have replied and said that it is outside their Money Back Guarantee period (which is 30 days from delivery), but have advised me to file a dispute with Paypal, where it is still covered. PayPal Buyer Protection lasts for 180 days (and covers the simple 'wasn't as described', which includes this situation, as a new item shouldn't be faulty).
I have contacted the seller and told them that, as a courtesy, I am giving them the opportunity to resolve this themselves, but that I will file a dispute on Tuesday if I haven't heard from them.