I would mention that if you employ contractors for the dig, don't let them back fill unless you've checked the depth of dig, the under duct sand and the duct joins cause they will nick depth and save time if you let them !
Very true. Leaving spoil along both sides of the trench makes it look deeper than it is when measured.
Have a couple of prepared depth sticks clearly visible when the contractors arrive to show you mean business. 150 mm from one end of the stick is painted red ( or any distinctive colour ) All that colour must be covered by sand when you probe along the trench before the duct is laid. A band at 600mm +150mm +diameter of duct from the end and that band must be at or below ground level when probing the depth of the trench. Another band can be used to ensure there is the right depth of sand above the duct before the earth is back filled.