This diagram shows the prescribed safe zones for concealed cables in walls.



If a cable is run outside of the zones shown with green and blue then it must either be buried to a dept of 50mm or more below the finished wall surface, or comply with regulation 522-06-06 parts (i), (ii), & (iii) (see below)




If the wall is 100mm or less thick then the safe zone also extends to the other side of the wall as shown here in more detail









Also an external link to have a browse through:

http://www.tlc-direct.co.uk/Book/4.4.1.htm
-Guide to the 16th Edition IEE Regulations 4.4 - Cable supports, joints and terminations: