You can contact the highest bidder, explain problem and offer to cancel their bid for them if they prefer. To contact them click on where it says number of bids on the actual listing or on your selling page. You'll get the list of bidders. Click on their ID and use the contact link top of next page.
If they want to cancel you can do that here:
http://offer.ebay.co.uk/ws/eBayISAPI.dll?CancelBidShow
Then you'd have to contact the next highest bidder.
