As the item supplied differs from the item advertised, I'd say you were in a strong position.
I've had disputes resolved using Paypal system, quickly, and recieved full refunds.
The seller has 100% feedback so bring it to their attention your intention to leave negative feedback (so damaging their reputation) if this is not resolved quickly and to your full satisfaction.
Finally, consider mentioning the Sale of Goods Act as the item is "not fit for the purpose it has been sold" as a last resort.
Hope this helps,