Although I can see your point with regards to wanting a refund for the item due to the fact that you've changed your mind, as a person who has sold on eBay myself in the past I can also see this from the seller's perspective.
As a result of your request to cancel the sale the seller is going to lose out on the PayPal fees relating to the sale, as unlike eBay PayPal will not refund the fees that they take in relation to a transaction in the event that the sale falls through. Therefore, I would suggest that you attempt to come to some kind of agreement with the seller with regards to the refund. The seller is likely to lose somewhere in the region of £2.00 in PayPal fees, so in order to come across as a buyer who is trying to strike a reasonable compromise suggest to the seller that he issue you with a refund of the money that you paid for the item, minus the fees that he lost to PayPal as a result of the fact that you changed your mind and asked to cancel the sale. If you were to do this then the seller may feel more inclined to issue a part refund, as it would show that you are willing to exercise a degree of flexibility with regards to the refund in order to compensate him for the loss of fees. If you start the e-mail off with something along the lines of "I apologise for any inconvenience that my request to cancel has caused you..." then the seller may well see the offer to accept a refund of the money less the fees that he lost to PayPal as an apology on your part and an attempt to try to smooth things over, as well as being an effort to try to work out a mutually acceptable compromise.
Given that you mention that the seller doesn't want you to leave him negative feedback I can understand the seller worrying about receiving negative feedback, as it can be very damaging to a seller's account. Therefore, if you were to include something in your message along the lines of "I can understand why you would not wish to be left negative feedback as it could impact negatively upon your feedback as a seller; however, I only wish to receive a refund and do not want to leave negative feedback" then this may well help to reassure the seller that you are not a classic example of the Buyer From Hell, as there is nothing in that sentence which could count as feedback extortion, in which case the two of you may be more likely to come to an acceptable compromise. An example of feedback extortion would be something along the lines of "Either you give me a full refund or I'll hit your account with some seriously negative feedback." This is against eBay policy and eBay would more than likely accept a seller's request to have any feedback of that nature removed in its entirety.
If you were to take the advice I gave you with regards to trying to work out a mutually agreeable compromise with the seller and this tactic did indeed work then you'd still get most of your money back, the seller would get back the money he lost to PayPal as a result of not having to refund the entire amount and if you're really lucky then when he's eventually calmed down he may well see this as a geniune mistake/change of mind on your part and refrain from adding your User ID to his Blocked Bidders List, which could turn out to be beneficial if at any time in the future you wished to purchase something else from this seller.
