167   Use of force

(1)  Reasonable force may be used for the purpose of gaining entry to any land (other than such part of a building as is being used for residential purposes) under a power conferred by this Division, but only if authorised by the roads authority in accordance with this section.

(2)  The authority:

(a)  must be in writing, and

(b)  must be given in respect of the particular entry concerned, and

(c)  must specify the circumstances which are required to exist before force may be used.