Access interfaces for managed private circuit networks; apparatus for linking computers into networks; apparatus for telephone networks; apparatus for use in communications networks; cabling networks (installations) incorporating fibre optic cables; communication devices for linking computer networks; communications networks; computer installations (networks); computer networks; computer programs relating to computer networks; data networks; data transmission networks; installations for use in forming computer networks; linked computer networks; local area networks; local optical fibre networks; optical networks; packet switching networks; receiving devices for connection to communication networks; telecommunications networks