Reference list:

Mental Health (Scotland) Act. (1995). http://www.legislation.gov.uk/asp/2015/9/contents/enacted

If there is an act number, as in the following example, include that also:

Resource Management Act, No. 69. (1991). http://www.legislation.govt.nz/act/public/1991/0069/latest/whole.html

The citation in your text will be:

(Mental Health (Scotland) Act, 2015)

or, if you have quoted directly, the section number

(Mental Health (Scotland) Act, 2015, s. 2)

If you have used the name of the act, including the year, in your sentence then no citation will be required, eg.

The Mental Health (Scotland) Act 2015 suggests that ...

or, if you wish to refer to a specific section,

Section 2 of the Mental Health (Scotland) Act 2015 states that ...