on by the person???(a) unconditionally or subject to such conditions as may be specified in the order or the arrangements;(b) for such period as may be so specified.(3) Arrangements made under an order under subsection (1)(d)???20(a) may be revoked at any time by the Forestry Commissioners;(b) do not prevent the Forestry Commissioners from exercising the function to which the arrangements relate;(