Or & finally.
Using a Y adaptor on the hose, with one side connected to the water, push the string into the other opening and into the water, the natural water pressure will push the string through the hose the same as if was unblocking it.
A similar method can also work by simply making a hole near to the start then again pushing the string inside, again the water pressure should force the string along the hose.
Bye.........
You would not get all the water out of the tube.!
Re.. "This way the inner cable is not constantly lying in water as it will if you use a cut hose"
Yet a further way .
Tie the end of the string to a mouse's tail, put it in the end of the tube, put a small piece of strong cheese in the other end of the pipe, wait, wait a bit longer, go and have a cup of tea, watch for the mouse, catch mouse while its eating the cheese and take off string, tie string onto the end of the cable, get someone to push in the cable in while its being pulled through the pipe from the other end, job done.
Trench dug to depth of 6", hose in trench, trench filled in and turf relaid... 
Not deep enough
! Its got to be at least a (If not ) more than a spade depth or for mains cables 450mm deep !
