Issue an Unpaid Item cancellation after 4 complete days to end transaction. More info:
https://www.ebay.co.uk/help/selling/getting-paid/resolving-unpaid-items-buyers?id=4137&st=
After that go to the dropdown actions menu alongside the item on your orders list and use the Second Chance Offer link to offer to the underbidder.
If the SCO link is not there (apparently it may disappear after cancellation) click on the original listing on your orders page, then click on where it says the number of bids and you'll see it there.
