From the Royal Mail site
"If at first we don't succeed we'll try and try again
We make every effort to make sure your mail arrives safely, so we'll keep it for three weeks (one week if you sent it by Recorded Signed For™). After that, we'll return it to you if you put your address on the outside, and we'll attach a sticker explaining why we couldn't deliver it.
If you haven't put your address on the outside, we'll send mail to the National Returned Letter Centre (or to Portsmouth for Special Delivery mail and packets, and to Belfast for letters). They'll open the mail and return it to you if your name and address is inside. If it's a newspaper, magazine or advertising, however, we'll dispose of it.
The last resort
If there is no return address available and the contents meet our storage criteria we may hold the items for three months pending claims via customer services.""