new out building

Since the run isn't too long (6m) and its for his own house I would be tempted to buy concrete coping stones (6"x1000mm) £3.50 each. and then lay this as an extra precaution. with the sharp sand. This Is what I what do if it were my housie.

 

I think a lot depends on where the cable is run.
If the cable is run under a lawn, it shouldn't need to be very deep as no one will be rotovating or turning over the soil.
If it's to run beneath a flower bed, then it would need to be deeper as people will be turning the soil over.
If it's to run under a drive way, then it will need to be deeper still and provided with additional protection.

 

http://www.pipes-underground.com/cable-ducting.html

 

Absolutely, right SpinLondon. I agreed with your point that it's totally depend on from where cable need to pass. I guess, you will able to fix it properly.