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:
Note: for the spacing of supports for cables having an overall diameter exceeding 40 mm, the manufacturer’s recommendation should be observed. * for flat cables taken as the dimension of the major axis. † the spacings stated for horizontal runs may be applied also to the runs at an angle of more than 30º from the vertical. For runs at an angle of 30º or less from the vertical, the vertical spacings are applicable.