Craig, The land drainage contractors association recommend a minimum cover of 450mm.
However, depending on the application and site conditions depths can vary
What is the application?
the application is for a large garden, the site conditions are very wet underfoot and if you start to dig the soil away the hole fills quickly with water
Can you post some photographs on the site? This will give a better idea of the site, type of soil, surrounding area etc
is the house in a new development?
Are ther other houses/gardens surrounding yours?