If an inspector is of the opinion that a person—
(a) is contravening one or more of the relevant statutory provisions; or
(b) has contravened one or more of those provisions in circumstances that make it likely that the contravention will continue or be repeated,
he may serve on him a notice (in this Part referred to as "an improvement notice") stating that he is of that opinion, specifying the provision or provisions as to which he is of that opinion, giving particulars of the reasons why he is of that opinion, and requiring that person to remedy the contravention or, as the case may be, the matters occasioning it within such period (ending not earlier than the period within which an appeal against the notice can be brought under section 24) as may be specified in the notice.