See
http://www.ais.org.uk/uk_aip/pdf/enr/2020205.pdf - para 7.5 - and it should explain things from the pilot side.
From the ATC viewpoint, the Shanwick / Shannon Letter of Agreement (LOA) states much the same thing so there is a double requirement (on the crew and the ajoining ATS provider)
Hope that helps, if not e-mail me for more.
RT