They will automatically return it. Providing they know who sent it. If there is a return address on the reverse of the package then it will be returned to the sender fairly promptly. If however there is no return address on the actual packaging then it will be sent to a specialist centre in Belfast where they will open the package and try to find out who sent it. This obviously may take a little longer. I know of one lady who had a package returned to her as she had wrapped the item in a Marks and Spencers carrier bag which had a receipt left in it. Royal Mail returned it to M&S who traced her through her account!
Some time ago, my local Post Office was insisting that packages should have the sender's name and address, or at least name and postcode, written on the back. They seem to have given up on this.
Sometimes the lengths post office go to are amazing. Many years ago I got a phone call from a sorting office, up north, saying that my christmas card recipient had moved away, so what did I want doing with it. This was only possible as I have included my phone number in it for some reason.
Goodness! The Post Office sounds quite intrepid! Fortunately my OH has put his return address on the back of the package so hopefully it should be returned to him sometime next week - thanks everyone for your replies.