It's illegal to use a metal detector anywhere without a license. The NATIONAL MONUMENTS (AMENDMENT) ACT, 1987 says that using one near a national monument or to search for archaeological artefacts is illegal, and that if you're caught using one anywhere the assumption will be made that you are using it to search for archaeological artefacts.
Here's the relevant parts of the act:
(1) Subject to the provisions of this section a person shall not—
( a ) use or be in possession of a detection device—
(i) in, or at the site of, a monument of which the Commissioners or a local authority are the owners or guardians or in respect of which a preservation order is in force or which stands registered in the Register, or
(ii) in an archaeological area that stands registered in the Register, or
(iii) in a restricted area,
( b ) use, at a place other than a place specified in paragraph ( a ) of this subsection, a detection device for the purpose of searching for archaeological objects, or
( c ) promote, whether by advertising or otherwise, the sale or use of detection devices for the purpose of searching for archaeological objects.
(6) Where in a prosecution for an offence under this section it is proved that a detection device was being used, it shall be presumed until the contrary is proved that the device was being used for the purpose of searching for archaeological objects.
(7) It shall be a defence for a person charged with an offence under subsection (5) of this section to show that the act constituting the offence—
( a ) was the subject of a consent under this section granted to him, or to a person in relation to whom he was acting as servant or agent at the time of the commission of the act, and in force at that time and was not a breach of a condition of the consent, or
( b ) was authorised by a prospecting licence granted under Part II of the Minerals Development Act, 1940 .
(8) In this section"detection device"means a device designed or adapted for detecting or locating any metal or mineral on or in the ground, on, in or under the sea bed or on or in land covered by water, but does not include a camera.