All you can do is ask the buyer to return the item and hope they're honest. As you've refunded upfront ebay will assume you don't want the item to be returned so you cannot issue a postage label through ebay for the buyer to use. If they agree to return it you'd have to send them a label via the Royal Mail website or see if they'd mind paying the cost of return and then reimbursing them via Paypal or similar.
In future even if you agree to them sending an item back always let the buyer open a return on ebay first. Sometimes they don't bother and you don't hear from them again. And no more upfront refunds.
@qajb02