Section 196A of the Town and Country Planning Act 1990 states:
"Any person duly authorised in writing by a local planning authority may at any reasonable hour enter any land—
(a)to ascertain whether there is or has been any breach of planning control on the land or any other land;
(b)to determine whether any of the powers conferred on a local planning authority by this Part should be exercised in relation to the land or any other land;
(c)to determine how any such power should be exercised in relation to the land or any other land;
(d)to ascertain whether there has been compliance with any requirement imposed as a result of any such power having been exercised in relation to the land or any other land,
if there are reasonable grounds for entering for the purpose in question."
Such a right exists without a warrant but, if entry is refused, Section 196B enables the acquisition and exercise of a warrant:
http://www.legislation.gov.uk/ukpga/1990/8/part/VII/crossheading/rights-of-entry-for-enforcement-purposes