The sum payable by way of a drainage charge in respect of chargeable land consisting of commercial woodlands shall be calculated as if the area of the land were one-fifth of its actual area.

In the application of paragraph 7(2) above to chargeable land to which sub-paragraph (1) above applies the area ascertained in pursuance of sub-paragraph (1) above (and not the area of which it is one-fifth) shall be treated as the area in relation to which paragraph 7(2) above has effect.