You can open an 'item not received' case. You don't need to log a complaint and may not be able to escalate if the tracking confirms delivery.
Generally, if the tracking shows delivery, Ebay will find in the seller's favour.
Make sure you check with neighbours and be prepared to chase Royal Mail yourself. Check the tracking to confirm the item is shown as delivered to the same place and at the same time as your other items.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)