If you are not at home, Royal Mail will attempt delivery of your item to one of your neighbours and will post a 'something for you' card through your letterbox telling you this.

If they're unable to deliver the item to you, or a neighbour, your item will be returned to your local Royal Mail Delivery Office for you to collect it, or to arrange a redelivery. Again, they will post a 'Something for you' card through your letterbox telling you this.