I have a small new build site lawn that is approx 5M x 3m sloping from 0 to 450mm. It is heavy clay soil that I want to level off, meaning I will have to build up 450mm. I am putting perforated drainage pipe on three sides to improve the drainage with outlet to a storm-water drain. I want to lawn the area with borders for flowers. What is the best way / combination of topsoil / aggregate / compost to make this level?