I have a very hard lump underneath my foot (where my three smaller toes are). the skin is hard but can be painful to walk on when pressure is applied.

A callous: This is likely to be a callous. See a podiatrist and have the foot evaluated.

Warty / Callous.: For all practical purposes, there're only two common causes for what you are describing. First is a callous caused by the skins reaction to increased pressure. The second is a wart infection. Warts are caused by a virus infecting the skin, often causing tissue hardness and sometimes discoloration. These conditions are treated differently. A specialist should make short work of the identification.