BS 7671 (2008 +A3), regulations 522.8.4 and 522, require that if conductors are not continuously supported, that they shall be supported at suitable intervals to ensure sure they are not damaged due to their own weight. The cable shall also be supported in such a way so as not to place any strain on any terminations.
The Onsite Guide to BS 7671, table 4a gives suggested maximum spacing for support clips: