There is a way to do this, as I did it a couple of weeks ago.
I also found it pretty hard to find, but eventually got there:
File and Unpaid Item Dispute here:
http://feedback.ebay.co.uk/ws/eBayIS...?CreateDispute
I know it's not a dispute, which is what got me confused in the first place too.
Once you're in there, there is a section that says that you and buyer have agreed not to continue with the sale. Select that one. When you do, it will send an email to the buyer asking if this is true. When they reply saying it is the case will close, and you will be refunded the final value fees you paid.
HTH