Sorry Tommy, English Law states nothing of the kind and we have had to delete your post as the information could cost a home owner a fiortune. To read the building regulations on foundations please go to our projects section where there are projects on both foundations and the building regulations. The dpeth width etc of foundations is determined by the ground conditions in relation to the load to be applied to them. In ALL cases the construction of a load bearing foundation is determined by the local authority. The nominal mix for a stable foundation is called C15P where P = Portland cement and is 1 cement, 2 sand and 6 course gravel. Please also look at our project on mixing concrete where we have a calculator to work out your own quantities.
A "lean" mix is simply a concrete mix which contains sufficiant cement to set but not to go as hard as rock. It is used mainly as a bedding material to stop the surface moving. An example of this is under slabbed pavements (not flexible block paving etc) which should not move at all. Lean mix concrete is very seldom used for structural work.