4(1)A person is an exempt individual on a particular day if—
(a)as regards the day he fulfils one or more of the conditions mentioned in sub-paragraph (2) below,
(b)at any time on the day he is severely mentally impaired, and
(c)he is stated to be severely mentally impaired at that time in a certificate of a registered medical practitioner.
(2)The conditions are that—
(a)he is entitled for the day to an invalidity pension under section 15 of the [1975 c. 14.] Social Security Act 1975;
(b)he is entitled for the day to a severe disablement allowance under section 36 of that Act;
(c)he is on the day of pensionable age within the meaning given by section 27 of that Act.
(3)A person is severely mentally impaired if he is suffering from—
(a)a state of arrested or incomplete development of mind which involves severe impairment of intelligence and social functioning, or
(b)an injury to the brain causing severe impairment of intelligence and social functioning which appears to be permanent.
(4)The Secretary of State may by order amend sub-paragraph (2) above as it has effect for the time being (whether by adding, deleting or amending conditions, or by any combination of those methods).
(5)The Secretary of State may by order substitute another definition for the definition of severe mental impairment for the time being effective for the purposes of this paragraph.