Electure,
As long as the conduit was sealed with a solvent cement I don't think there would be a problem.
As Bjarney said, there are cables made to run U/G, but I would personally keep them away from any mains voltage cables, over any long length of run.
Over here, a Telco, stipulates that the power and thier cables must be vertically segregated(in a trench) by 300mm, this is the same for any other "services" (water, etc) that go into the same trench. [Linked Image]