(1)A person authorised by a local housing authority or the Secretary of State may, at any reasonable time, on giving 24 hours' notice of his intention to the occupier, and to the owner if the owner is known, enter premises for the purpose of survey and examination—
(a)where it appears to the authority or Secretary of State that survey or examination is necesary in order to determine whether any powers under this Part should be exercised in respect of the premises, or
(b)in the case of premises which the authority are authorised by this Part to purchase compulsorily.
(2)An authorisation for the purposes of this section shall be in writing stating the particular purpose or purposes for which the entry is authorised.