An officer may give such an authorisation only if satisfied on reasonable grounds that the use of those powers in the locality during that period may be necessary for the purpose of removing or reducing the likelihood of—
members of the public in the locality being harassed, alarmed or distressed, or
the occurrence in the locality of crime or disorder.
An authorisation under this section—
must be in writing,
must be signed by the officer giving it, and
must specify the grounds on which it is given.