IAML (UK and Ireland)* has posted useful information on their website about this. Read it HERE.
'29 October 2014 saw the implementation of legislation to facilitate the use of orphan works in the UK. An orphan work is defined as a copyright work or performance for which one or more of the rights holders either cannot be identified or cannot be located, thus making it impossible to seek permission for use of the work.
Use of orphan works is now permitted under two different routes: the implementation of the EU Directive 2012/28/EU, or under the UK Licensing Scheme.' Read more HERE.
*IAML is the International Association of Music Libraries. We're in the UK and Ireland branch.