A person authorised by the 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 purposes of survey and examination where is appears to the authority or the Secretary of State that survey or examination is necessary in order to determine whether any powers under this Part should be exercised; or
for the purpose of survey or valuation where the authority are authorised by this Part to purchase the premises compulsorily.
An authorisation for the purposes of this section shall be in writing stating the particular purpose or purposes for which the entry is authorised.