As long as tracking says delivered, then you'll win any case the buyer opens for item not received, just ensure you have added that tracking number where you marked the item as dispatched.
Send the buyer a message including the tracking number, ask them to go to Royal Mail's Track & Trace online. Entering that tracking number will show them where and when this item was delivered. It might be with a neighbour, in a safe place, or even in the Sorting Office ( not Post Office), awaiting collection or instruction to re post the item.
